"+Paul clearly states that we are not under the Law but under Grace. Although Christ never taught that..."
Well, that's not quite true. For example, Christ repeatedly "broke the Sabbath." He even used a specific example of David eating something he wasn't supposed to eat (the blessed bread in the tabernacle of the Lord) to illustrate his point that "the Sabbath was made for man, and not man for the Sabbath."
Given the centrality of Sabbath-keeping in the law, this alone was a huge statement by Christ regarding the law.
You are right that the Church ended up setting dietary restrictions, and I suppose that there is some irony in this, but in the Christian era they have a different character, a different motivation, and a different goal.
In the Old Testament, God was teaching a basic concept of "clean" and "unclean" -- some of this may have been determined by the associations certain foods had in the pagan world, and in Egypt in particular. We don't know. Part may have been just the beginnings of teaching self-control and separateness from the pagan world in general. I don't know.
In the New Testament era, our fasting is of a positive, ascetic nature. It is also a specific and direct reaching back to Paradise, where we didn't use animals as sources of food. But as far as I know, there are no foods that are completely banned at all times (other than blood, which is specified in the New Testament as being off-limits.)
To a great extent, Orthodox fasting guidelines are a formalization of the kind of ascetic fasting that probably took place spontaneously and through the effect of oral tradition from the Apostles.
But He never said that His followers were not under the Law but rather under Grace, did He? The case of Sabbath "breaking" served to teach that greater good is always the side to "err" on, not that those who followed Him or His teaching encouraged breaking or abandoning the law.
In the New Testament era, our fasting is of a positive, ascetic nature. It is also a specific and direct reaching back to Paradise, where we didn't use animals as sources of food
Our fallen nature is incompatible with that kind of diet. We now know that essential amino acids do not come from plants, or not in sufficient amounts. I am sure when we (or at least some of us) are in Paradise, eating herbs will be quite sufficient.
I disagree that this was the reason for dietary abstinence. Christians, who were originally mostly Jews, fasted as a matter of their Old Testament practice. At one point it became erratic, and the Church regulated the extent and type of fasts which differed for monastic ranks (they were longer). Eventually, the Church settled into the current fasting practices as a matter of conformity to the faith, as defined by the Church.
I think my example of +Symeon the New Theologian shows that fasting is not necessarily the requirement for salvation but what's in your heart, how Christ-like we are.
The bottom line is that the current fasting practices prescribed as required part of being faithful are man-made tradition and is not based on anything Christ specifically taught, or commanded.
If anything, fast is a form of sacrifice, self-imposed humility, self-denial, a discipline so important to reaching higher levels of spirituality because self-denial allows the Spirit to work in us; the humbler we become the less arrogant and proud we become, and more Grace-filled.
That discipline is known to all eastern religions and for the same reason.
Last but not least, Jesus never even raised the possibility of abolishing circumcision. +Paul did. But in order for him to make that concession to the Gentiles, he had to replace the law with grace.