Did Buddha Return to Extreme Asceticism?

The Buddha advocates a life of mild, in the middle, asceticism but that doesn't mean a practitioner can't practice in an extreme way when it is deemed appropriate to do so when the practitioner knows well the middle way foundation.

There are core teachings of the Buddha's that he repeated many times in many different ways. Look to those core teachings to discern for yourself what is the real teaching when the teachings are seeming ambiguous and possibly corrupted.

There was a conflict with (Buddha's brother in Law(?) or cousin) Devadatta (see the Devadatta-suttas). Devadatta tried to become chief of the sangha instead of the aged Buddha; and to collect adherents/followers he claimed 5 (?) stronger ascetic rules. Because there was this conflict at all, thus the Buddha must have had milder ascetic rules and according to the transmission in the suttas, he had refuted that radicaler asceticism rules (but allowed them as individual training). So I would put the answer to your question

Did Buddha Return to Extreme Asceticism?

saying no .
Which does not mean that he wouldn't have left the leading disciple Mahakassapa in peace when this old man went aside into the forests to stay alone and feasting as he wished (see the nice Gosinga-forest sutta). He allowed a spectrum of behaves - as long as that specific behave leaded to emancipation, liberation and ceasing of dukkha.

These different descriptions, depended on what level you are able to understand, are all correct, or valid. The abstaining from food to starve like only a pair of skeleton - the practice of White Skeleton visualization, they have intrinsic meanings. The consumption of rice-milk - the Great Ch'an Patriarch Huineng said it's in fact the Milk of Vairocana. The full practice of White Skeleton kept in one of the Chinese Sutras translated by Kumarjiva has similar analogy. Vowed to sit under the Bodhi tree until enlightened else never rose from the seat, indicates but his determination, not a self-torture. I don't see there is anything what you termed Extreme Asceticism before or after.

Now in the Tibetan tradition, there is part of similar progress of Milarepa's practice. He dwelt in the mountain years (7 years?) without clothes and foods, eating only nettle leaves. On the verge of dying he opened his teacher's sealed bag, in it held the last words of his deceased teacher only can be read at emergency, it said "good food is the key (to pass the final stage)".

Abstain from food is not (aimed to do) a self-torture or to weaken the body, what's the use these to enlightenment by inflicting pain on oneself and with a weak body + bad health?? It's a practice, it's part of the whole enlightenment process. I don't think it's simple to explain here with so many terms and concepts not available in general understanding.

I've never read anywhere about 49 potions for the next 49 days or He then got up and went begging in a village, are you sure this is correct and authentic? There are too many incorrect stories floating in the internet, I suggest you read the original Sutras if possible, not too rely on those articles written by some who-know-whom.

The story you're recounting comes from the Jataka tales.

I think some of these tales may be exaggerated or fanciful -- see Does Theravada Buddhism accept Jataka Stories? -- or metaphorical (e.g. as Thiago wrote the "vow" implies resolve, not ascetism).

What may be a more reliable (and less supernatural) account is in the Ariyapariyesana Sutta (MN 26), which for example I mention in this answer and based on which I concluded "no asceticism".

"No asceticism" is consistent with SN 56.11, when the Buddha taught the doctrine of the "middle way".

No. The Buddha did not return to asceticism or ever recommend asceticism to his monks.

The phrase you are citing is found in many suttas & refers to the arousal of energy.

Āraddhavīriyo viharati: ‘kāmaṃ taco ca nhāru ca aṭṭhi ca avasissatu, sarīre upassussatu maṃsalohitaṃ; yaṃ taṃ purisathāmena purisavīriyena purisa­pa­rakka­mena pattabbaṃ, na taṃ apāpuṇitvā vīriyassa saṇṭhānaṃ bhavissatī’ ti

Here, a bhikkhu.. has aroused energy thus: ‘Willingly, let only my skin, sinews and bones remain, and let the flesh and blood dry up in my body, but I will not relax my energy so long as I have not attained what can be attained by manly strength, energy and exertion.’

AN 8.13; SN 12.22; SN 21.3

By this point, the Future Buddha had tried extreme mortification, realized that wasn't working, returned to eating to sustain his life, went under the Bodhi Tree, then makes the vow to not leave again until he finds the answer, EVEN til his body is dried and shriveled.

The ascetic practices he did were the methods he thought would lead to some spiritual attainment. He thought that not eating and making his body weak could have some benefit.

Going to the Bodhi tree and not leaving is not a method. He didn't think that the act of not leaving the shades of a certain tree was a particularly interesting practice on its own merit.

The vow is interpreted not as asceticism, but as resolution or commitment. Whether he really meant that he would die under the tree we can't tell, but by the time he took food, it's clear that he was giving up the idea of trying to inflict pain in his body as a means to spiritual progress. Finally, it's clear that the practices he undertook under the tree were not ascetic in nature.