Why is the boat form of paraldehyde not theoretically possible? Wikipedia says the following about paraldehyde: 


  Theoretically, four stereoisomeric structures are possible. The
  structures (1) and (2) are known as cis- and trans-paraldehyde. The
  structures (3) (a conformer of (2)) and (4) (a conformer of (1)) don't
  exist for steric reasons.
  
  


If this is correct, it seems that the boat conformer is not theoretically possible. I would expect the boat conformer to be possible but be much less common than the chair conformer (as is the case with glucose and cyclohexane for example), but that does not seem to be the case.

Wikipedia says that conformers 3 and 4 don't exist for steric reasons, however they are still theoretically possible. To me this sounds like the nonexixtence of boat conformation is not just because of steric reasons, but there is a more fundamental reason for that.

So, my questions are:


Is it true that the boat conformer is theoretically impossible?
If yes, why is it impossible and what makes paraldehyde different from glucose and cyclohexane in this regard?
A $\ce{CH_3}$ group is quite big. Steric hindrance is the reason. 

Its not theoretically impossible, but it is energetically a lot less favourable than (1) and (2). 

You wouldn't find it in nature so to speak.

to 2: A $\ce{OH}$ group is sterically a lot smaller than a $\ce{CH_3}$ group.