20 ball rallies at 3.5 are pretty damn rare. They are rare, in fact, at the highest levels of the game. I don't think it's realistic to expect that kind of patience, shot tolerance, and point construction from a player at this level.
I know players that look like 4.5s who are 4.5s. And I know players that look like 2.5s who are weak 3.0s. I do not, however, know any players who look like 5.5s who are from Iceland. I'm sure they are out there; I just don't know any.
From my experience playing mixed, and I have also heard this from several other people, it is spin and not pace that mostly separates the men's game from the women's at similar NTRP levels. Heavy topspin on groundstrokes and serves was, from my observation, almost uniformly unreturnable for 3.5...
Whey is fine unless it irritates your digestion. It's almost impossible to harm yourself by ingesting too much protein as such (clearly this doesn't apply to red meat). More than the average person gets appears to be needed for optimal response to resistance training.
Anyone who uses a one-handed BH with big grips on either side will have different return mechanics than a two-hander who mostly doesn't have to switch grips. Federer hits over the ball more on his returns because his grips aren't as extreme and he has better hand-eye coordination.