What are the benefits of keeping the lay precepts, if you're not a bodhisattva?

It's basically for laypersons to reduce suffering, even if not to eliminate suffering altogether.

The Vera Sutta below states that the five forms of fear and animosity are stilled through the observance of the five precepts.

Interestingly, the sutta does not say that the five precepts lead to stream entry. Rather, it says that reaching a certain state of mind i.e. the stilling of the five forms of fear and animosity (which correspond to the five precepts) would lead to stream entry.

If you lie or steal, you would be afraid of being caught. Animosity is with those who could catch you. Observance of the five precepts results in a state of mind free from such fear and animosity. Additionally, virtues also result in a state of mind free of remorse, which is discussed in the Kimattha Sutta (AN 11.1).

From the Vera Sutta (AN 10.92):

Then Anathapindika the householder went to the Blessed One and, on arrival, having bowed down to the Blessed One, sat to one side. As he was sitting there, the Blessed One said to him, "When, for a disciple of the noble ones, five forms of fear & animosity are stilled; when he is endowed with the four factors of stream-entry; and when, through discernment, he has rightly seen & rightly ferreted out the noble method, then if he wants he may state about himself: 'Hell is ended; animal wombs are ended; the state of the hungry shades is ended; states of deprivation, destitution, the bad bourns are ended! I am a stream-winner, steadfast, never again destined for states of woe, headed for self-awakening!'

"Now, which five forms of fear & animosity are stilled?

"When a person takes life, then with the taking of life as a requisite condition, he produces fear & animosity in the here & now, produces fear & animosity in future lives, experiences mental concomitants of pain & despair; but when he refrains from taking life, he neither produces fear & animosity in the here & now nor does he produce fear & animosity in future lives, nor does he experience mental concomitants of pain & despair: for one who refrains from taking life, that fear & animosity is thus stilled.

"When a person steals... engages in illicit sex... tells lies...

"When a person drinks distilled & fermented drinks that cause heedlessness, then with the drinking of distilled & fermented drinks that cause heedlessness as a requisite condition, he produces fear & animosity in the here & now, produces fear & animosity in future lives, experiences mental concomitants of pain & despair; but when he refrains from drinking distilled & fermented drinks that cause heedlessness, he neither produces fear & animosity in the here & now nor does he produce fear & animosity in future lives, nor does he experience mental concomitants of pain & despair: for one who refrains from drinking distilled & fermented drinks that cause heedlessness, that fear & animosity is thus stilled.

"These are the five forms of fear & animosity that are stilled.

AN 5.57 (below) teaches that a lay person or even a monk or nun should take responsibility for their actions, and this will diminish their misconduct. This is related to the five precepts too.

“And for the sake of what benefit should a woman or a man, a householder or one gone forth, often reflect thus: ‘I am the owner of my kamma, the heir of my kamma; I have kamma as my origin, kamma as my relative, kamma as my resort; I will be the heir of whatever kamma, good or bad, that I do’? People engage in misconduct by body, speech, and mind. But when one often reflects upon this theme, such misconduct is either completely abandoned or diminished. It is for the sake of this benefit that a woman or a man, a householder or one gone forth, should often reflect thus: ‘I am the owner of my kamma, the heir of my kamma; I have kamma as my origin, kamma as my relative, kamma as my resort; I will be the heir of whatever kamma, good or bad, that I do.’

Precepts serve two purposes:

Purpose One is training the mind by changing your behavioral/emotional habits.

Purpose Two is improving one's karma by not creating turmoil in one's future that comes from the misdeeds.

In the long run One and Two help each other to get stronger. Together they gradually propel one further and further into Liberation.

Even if you take a liberal interpretation of the precepts you will train your mind and improve your karma.

In any case you should start from the most coarse offenses creating the biggest problems before proceeding to the subtle mental habits and misdeeds creating more subtle forms of trouble and suffering.

For example, being easily irritated and getting into fights all the time is a source of coarser and more obvious problems than killing mosquitoes or eating chicken and fish. So you should start with the first.

For example, craving sex to the point of rapping someone or seducing someone's spouse and breaking their family is a source of far coarser and more obvious problems than indulging in observing people of the opposite gender at a beach, so again it makes sense to start with the coarser one.

For example having a fragile ego and pathologically lying to cover up one's mistakes is a source of coarser and more obvious problems than not phrasing one's statements precisely enough to make sure they are absolutely true.

And as you correctly assumed, having a complex emotional trauma leading to a drug or an alcohol addiction is a source of much coarser and more obvious trouble than intoxicating one's mind with videogames, not to speak of indulging in an occasional beer.

From this it should be obvious that Buddhist path should be approached from coarse to gradually more subtle - by anyone and everyone, monk or lay.

If you think about it, the Eightfold Path gets into more subtle matters than mere Five Precepts, and the Four Jhanas get into more subtle matters than the Eightfold Path (i know they are included but you get the point). Within the four jhanas, the first jhana deals with coarse issues the second is more subtle etc. Entire Path is structured like that.

Obviously, someone who professionally pursues Anuttara-Samyak-Sambodhi is more likely to go all the way to perfection of practice, while someone engaging with Buddhism casually among other things will probably stop at preventing the most obvious troubles. How far to go is entirely up to an individual. The benefit is proportional to implementation, to whatever degree one eradicates potential causes of dukkha - to that degree one is liberated from it.

To summarize, it's not about liberal vs strict implementation of precepts, it's about coarse vs subtle causes of dukkha - similar concepts but not the same.

In answer to your direct question in the title:

What are the benefits of keeping the lay precepts, if you're not a bodhisattva? If you are a bodhisattva, you don't need to keep the precepts because you have no inclination ever to act otherwise. But if you are not a bodhisattva, you'll never get there in this life unless you keep the precepts:

The reason for practicing dhyana and seeking to attain Samadhi is to escape from the suffering of life, but in seeking to escape from suffering ourselves, why should we inflict it upon others? Unless you can so control your minds that even the thought of brutal unkindness and killing is abhorrent, you will never be able to escape from the bondage of the world's life. No matter how keen you may be mentally, no matter how much you may be able to practice dhyana, no matter to how high a degree of Samadhi you may attain, unless you have wholly annihilated all tendency to unkindness toward others, you will ultimately fall into the realms of existence where the evil ghosts dwell. Pure and earnest bhikshus, if they are true and sincere, will never wear clothing made of silk, nor wear boots made of leather because it involves the taking of life. Neither will they indulge in eating milk or cheese because thereby they are depriving the young animals of that which rightly belongs to them. It is only such true and sincere bhikshus who have repaid their karmic debts of previous lives, who will attain true emancipation, and who will no more be bound to wander to this triple world. To wear anything, or partake of anything for self-comfort, deceiving one's self as to the suffering it causes others or other sentient life, is to set up an affinity with that lower life which will draw them toward it. So all bhikshus must be very careful to live in all sincerity, refraining from even the appearance of unkindness to other life. It is such true hearted bhikshus who will attain a true emancipation. Even in one's speech and especially in one's teaching, one must practice kindness for no teaching that is unkind can be the true teaching of Buddha. Unkindness is the murderer of the life of Wisdom. This is the second admonition of the Lord Buddha as to the keeping of the Precepts. (The Surangama Sutra, Chapter 2, "Importance of Keeping the Precepts)

As you point out in the background to your question, what 'keeping the precepts' means, unfortunately depends upon who you ask nowadays. The crucible in regards to the so-called 'Lay' Precepts—which all ordained sangha members must also keep, so they aren't ‘lay’ precepts, they're the Big Five Precepts—is the consumption of animal flesh, which, of course, entails killing the animal. I’ve answered this point here: Why is meat prohibited in Buddhism? So I won’t repeat what I said there, but in side-comments to my answer an important point came up:

But as a Buddhist, one should adhere to the doctrine of causes and conditions, and realize that eating the flesh of animals creates the possibility for a profitable demand for animal flesh in those of weak minds who are unconcerned about profiting off the suffering of others for their own benefit. By eating animal flesh, you may sincerely feel that you are not directly responsible for the animal’s death, but by creating the condition for someone of weak mind and strongly selfish desires to cause harm to others, you are responsible for leading them astray. One may try to abdicate their responsibility to protect the weak and voiceless, but one cannot escape responsibility for their own actions that harm others, that lead them, through their weakness, to the hell realms.

The point here is that there is no honest wiggle-room here for any sincere buddhist, ‘lay’ or ‘ordained sangha’. If you make excuses for your behaviors—liberal interpretations, versus conservative, as you phrased it well—you are hurting yourself over the longterm, along with others who suffer due to your weakness of spirit.

Anyway, that’s my interpretation ☺️