I couldn't find a rule that prevents either of these, but just to be sure:
1. If a wall is created in front of a portal, can you attempt to damage the wall from the other portal entrance? (in other words, can you cast spells that require LoS across portals?)
Yes. The two squares connected by a portal are treated as adjacent for all practical purposes.
2. Can you boost counter spells?
Yes, as long as the spell explicitly allows it.
ps: am hoping to have my first play tonight.
Have fun playing.