I have read some theories and one of the more modest estimations was that 1kg makes up for roughly 30 seconds on a 30 minute climb. That means the difference between, say 61 and 63 kg, gives a 1 minute difference on a 30 minute climb. If you only count the MTF in the Vuelta and the time it would take to climb them, this would mean losing plenty of minutes over 3 weeks. Plenty more than he can ever dream of making up in the TT.
Now if you take into account the other -not MTF- climbs, you also realise you have to push bigger numbers and go deeper constantly, wasting more energy and tiring yourself more over the course of 3 weeks. This is actually what worries me most, especially with his inconsistency the past two seasons. You can have killer legs, if you don't have the energy anymore to push those watts to compensate for those extra kg, you shut down. So it's possible everything goes well for 2 weeks, and then suddenly the lights go out.
Obviously this theory is when you don't have to lose leg muscle but just fat and can still produce the same watts as before. Remco, at the moment looks relatively sharp (looking at instagram and twitter images from the past days), so i don't know if he could lose an entire kg of only fat, or at which point he would start losing muscle mass. But even 500grams actually make a difference each day. There is a reason climbers don't have legs like sprinters. So him being heavier in relation to his weight, than the least lanky GC guys of the past decades causes quite a bit of doubt for me. Especially with a team that frankly often gives the impression it doesn't seem to know what they are doing, making stuff up as they go and trying a completely different approach compared to last year, based on the assumption that this was the problem and not his recovery and preparation.
For me personally, it is the biggest concern. Whether this is the right balance between staying healthy (not getting sick), power output, climbing, and lasting 3 weeks. If it is, i applaud his trainers, but considering they are using his weight as a scapegoat for his Giro failure, considering he appears to be heavier (in relation to his height) than any successful GT rider of the past 15 years, considering the team is not experienced at all in these matters (unlike other teams), i can only hope they got it right.
I agree with your point about the team lacking the necessary knowledge but as far as his ideal weight it's just speculation honestly. It may well be the case that he performs better being slightly heavier rather than being as skinny as he was last year. Sure if he could lose 3 kg without losing any power that would certainly be a big improvement but that may not be possible for him.
The way he has been climbing this year is much better than last year and probably on par with his first year as a pro. Even last year in the giro after a terrible preparation after his injury he still did pretty well for the first 2 weeks which makes me think we should have higher expectations for this Vuelta. There is no way he isn't in significantly better shape than he was last year. But yeah, his team doesn't fill me with confidence either.