Why is the boat form of paraldehyde not theoretically possible?
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.