>1) Is it possible to replace the valve cover gaskets on a '78 2.0L 
>with the engine in the bus?  

YES. 

>2) Is it possible to replace the pushrod tube seals with the 
engine in.

YES.  Unlike with a type 1 motor, doing this with the engine in is 
not difficult. You don't have to loosen the heads, etc. Refer to 
the Bentley or Haines manual. 
>
>3) I imagine it's completely out of the question to replace the 
>oil cooler gaskets with the engine in, at least not without a LOT 
> of disassembly. True?

It depends on what you mean by a lot of disassembly.  It is an 
afternoon's work.  You have to remove the fan shroud, but that's 
about it.

-Ron Salmon