Explaining non-attachment to the very attached

"but my specific question is, what to do when Dharma is (un)intentionally violent like this?"

Practice the Five Faculties during and outside of meditation. The Five Faculties are Mindfulness, Concentration, Faith, Energy, and Understanding.

These are in no particular order and are like the wheels on a cart: if any of them are weak, idiotic extremes occur: mass genocide, stagnancy, zealotry, etc.

To be honest your question shows you need to actually study Dharma, at least read a few Buddhist works, preferably one old text and another more modern like "Mastering the Core Teachings of the Buddha" which is free online.

I would suggest having faith that Buddhism works for many of us and studying it further. It is not easy to get everything in life figured out but Buddhism makes it possible. Also the Buddha was not an "idiot" for his decision. At the very least, Gautama Buddha was a brave man and an opportunist with a good eye for what will eventually happen (death for all) and what excellent thing he can search for (Awakening, liberation from death) for the most benefit of all for all time. He is the great hawk, your girl friend is the blind bat. Life is never perfect and sometimes extremely difficult sacrifices must be made for the highest good.

"How can you smile?" -- I'm smiling simply because I'm happy!

Perhaps this is a case where it's best to simply say, "This works for me, and is important to me, and I mean no harm to you by my practice," and walk away. I don't know if "walk away" is an option in this relationship, but if this person is having this hostile a reaction to your practice, then I don't know how you could resolve it in a way that will help the person. I think the best you may be able to do it maintain your practice, remain an example of the peace and happiness that it can bring (smile!), and let the person tread their path.

"How can I help or at least not hurt these people in my life?"

"Don't ask me that"

... said the Buddha to people asking him questions that would torn them. Following his example, we should not be careless towards others -- "right speech bullet lists" is just not enough.

For example, it's not wise to alienate someone who regard us as dear by selectively implying that the feeling might not be reciprocate (say, when talking about "non-attachment"). It's not fair to expect them to not be hurt, specially when we are the ones who study so intensely the causes of suffering (e.g. "being apart from what is desirable"). And specially when we actively participated in creating that very "attachment" in the other.

Another example would be when one uses "non-attachment" as an excuse for "non-commitment", like a green card to be forever unreliable. The list of misuses and misunderstandings goes on. But the point, I think, of the quote above is we should ponder carefully about the benefit (or injuries) our actions and words bring to others. And our responsibility in these acts.

"If one regards himself superior or equal or inferior by reason of the body that is impermanent, painful and subject to change, what else is it than not seeing reality? Or if one regards himself superior or equal or inferior by reason of feelings, perceptions, volitions or consciousness, what else is it than not seeing reality? If one does not regard himself superior or equal or inferior by reason of the body, the feelings, perceptions, volitions or consciousness what else is it than seeing reality?"

— SN 22.49

From the above, we should not regard, imply or entertain such comparisons. Some of us might regard the dhamma as perfect, and a path to perfection. But what is the use of such expressions when it brings people to dwell in these very same comparisons? Fundamentally, the Buddha dhamma is about "relieving suffering". Isn't it enough when approaching what we are doing when following the dhamma? Is there any chance for one to be at blame for standing by this?

The best option would be to keep following the noble eightfold path and observe the different path factors and the 5 precepts. This way you have your side of the coin clean and you are not pushing or preaching to her in any way.

If she get's angry when seeing you meditate then that is her own doing. The untrained mind does not see reality for what it is. Instead it extrapolates and identifies with objects. In her experience she thinks you are doing something to her where in reality she is creating her own suffering. But how to explain that?

It can be very difficult to explain or talk about the Dhamma to non-buddhists and especially non-meditators. The reason for this is the practical nature of buddhism and the emphasis on experiental knowledge instead of intellectual knowledge.

I often tried this and have begun using the "fruit-analogy". Imagine one has brought you a fruit from a country far away. You have never tasted the fruit and are curious about it's flavour. The bringer of the fruit tries to explain the flavour of the fruit to you. He uses all kinds of adjectives and descriptions but you can never fully understand the flavour of the fruit before taking a bite and tasting it for yourself. So it is with some buddhist doctrines. They are so profound that they cannot be fully grasped by the intellect.

This analogy usually puts people to sleep, i.e. ends the discussion.

When that is said then some people will just never understand the choices and ways of life that other people practice. In these cases one must ask oneself - how much time and energy should I spend on explaining it then?

How can I help or at least not hurt these people in my life?

By keep practicing the noble eightfold path. Keep observing the precepts. Keep meditating and thereby puryfing yourself from the defilements. When doing that only pure intentions will come from you. When you are following the noble eightfold path your intentions will be; intentions of renunciation, good will and harmlessness.

When not having any unwholesome intentions then you are not hurting anyone. If anyone is getting hurt they are hurting themselves. In that case you can offer your advice and help. A good explanation could be to explain how human beings superimpose qualities onto objects, e.g. one experiences grey and rainy weather and says it's "sad". The weather is not sad. The weather is neutral and objective. It is human beings themselves who put the quality of sadness onto the weather thereby making themselves sad.

The fruit- and weather-analogies are good ones to use because they are simple. They are non-preaching and they are understandable also for non-meditators.

I've been told by someone close to me that my meditation and desire to let go are very threatening to her. She sees sense desires, ego and attachments as very natural and wholesome, and my way of life as a threat or a questionable choice at the very least.

If you mind your own business, and you don't preach/push other people into your way of thinking, it's entirely her decision to feel threatened. it is important that feeling and being are different things.

If, on the contrary, you are trying to sway her towards your beliefs, then she might be right.

[...] thinks the Buddha was an idiot for leaving his family and palace to chase nirvana [...]

It is entirely up to her to think about whatever she wants in whatever way she wants. If you don't like that, then you are attached to the idea/facts she's against.

Seeing that I was very cheerful after an hour long harangue only infuriated her some more.

It is completely normal. Most people are infuriated when their aggressive response goes nowhere and they do not get the reaction they expected.

They way you behave is also important. If you are simply peaceful and non-imposing, it is good. If you even seem to be patronizing, you should work on that. Having different beliefs does not make you superior (and neither it makes her so).

By meditating all the time you are trying to be perfect, you're actually making fun of us who don't meditate and like our egos. I get angry when I see you meditate.

This is twofold. You should take time to explain her that what you are doing is your choice. You are not doing it be better than others - but because you feel that it is better for you. To give a vivid comparison: if I say that try to learn to drink ten pints of beer at one sitting because I like it drunk, it is one thing; if I say I do that to be better than others at drinking and will consider them inferior, it will be outright rude.

Second part is that she is angry when she sees you meditate. It is, again, entirely her decision to become angry.

How can you smile? Stop treating me like a child.

This strongly hints that your communication leads her to think that you are patronizing. It takes two to cuddle, so you have to take steps to prevent that and communicate better.

what to do when Dharma is (un)intentionally violent like this?

It is not violent by itself. If you communicate with people well, they will usually leave you to your own devices, whatever they may be. Threat comes from the expansion on the views when you are trying to spread them. Be gentle: if a person doesn't want to know the details, don't push them.

How can I help or at least not hurt these people in my life?

Frankly, people do not need your help. If you think that way, then it is partially a source of the trouble. You are doing things differently to them. If you explain that plainly, without arguing that you are doing things better, it will be the first step towards normal coexistence.