Siege - I think your theories are all pretty sound. I must confess to watching some of the program on monday and my thoughts there where
pretty much
a) was that really blaine? I mean he looked similar to the guy who did the street magic shows - but he also definitely looks a little different - so yes - my guess is the changes are for a swap at somepoint.
b) There are just so many ways he could have smuggled food supplements in that it isn't even worth considering that he isn't eating.
"Space Food" could be hidden in the supports in the box,
"Space Food" could be hidden in his clothes
"Space Food" could be hidden in his belongings
"Space Food" could be hidden in his false stomach (I blame yankee toms false skin for this

)
and/or of course - the water could contain a glucose...
I suspect though tht 44 days is too long for him to be happy - so I agree with you that switches will be made.
I then thought about how he would do that - and then decided that I didn't really care. He will do it though, I'd put money on it that it happens.