Yes you can. I think, but i'm not 100% sure, that the b18b head is just alittle better. Not to the point where you would notice a difference though.
I have a B18a head on a B18b block. My last head had a crack
The differences between the two heads is a slight change in either the intake or exhaust cam (not sure which). Most of the changes were in the ECU mapping.
They are completely interchangeable, and you wouldnt notice a difference between the two.
Positive Crankcase Ventilation. You had it right the first time
and as Dylan mentioned, blocks are pretty much the same, everything is interchangeable between the two motors. It isnt a performance upgrade though, if you arent replacing the block because theres something wrong with it.