I actually happen to agree with Twin on the weight of the armour, because the problem is that while it may well be powered, insofar as it's visually depicted it isn't self-supporting - which means that the wearer has to absorb its full weight on their shoulders, spine, and legs. I guess we can handwave this a bit by saying that the undersuit is actually self-supporting and capable of holding the weight of the armour, meanings the Spartan inside just needs to 'drive' the thing rather than actually holding it up and pushing it around with their own muscles, but even then, it would be nice if there were some visual elements which actually demonstrate this. Put bluntly, unless it contains its own synthetic muscles, any clothing or armour is going to have a negative impact on the strength and agility of the wearer, and Spartans are no different.
In terms of the medical stuff? I can accept it because it's the future. We have medicines and procedures today which scientists even a hundred years ago would have dismissed as completely ridiculous (pacemakers, organ transplants, vaccines, etc), so while it may seem impractical or impossible to us today, it's reasonable enough to believe we'll be able to do it in 500 years.
As for the heat resistance - your physics is flawed, Twin, we have stuff that can do that today. And given that current Mjolnir was specifically built to resist plasma attacks - which are even hotter - the only issue with falling from space would be the force of impact, not heat. I also think you're probably overestimating the amount of armour which is dedicated to stopping projectiles - remember, as we saw with Kat, Mjolnir armour can be quite easily pierced once the energy shielding is down, so it's probably safe to assume that the majority of the armour we see is there to help give the shield its shape or absorb heat, rather than resist bullets. Taking that as true, and assuming the undersuit is in fact essentially a self-supporting synthetic muscle which interfaces directly with a Spartan's nervous system rather than just to cover their modesty, I think Mjolnir as a concept is perfectly within the bounds of realism, even if it is centuries ahead of where we are technologically.
OT: I don't know nearly enough about metallurgy to even begin to speculate.