Scenarios don't have entrance portals and when we get WoD I imagine we cant que for MoP Scenarios (similar to dungeons) so what happens if say we want to go back and get the achievements for them?
Probably nothing in the short term. I imagine long term they will implement that level scaling feature so you can go back and do past lfrs/scenarios/etc.
Thats actually a good question. Although you can talk to some NPCs that will queue you to the scenarios in the general area where they happen. Not sure thats how it will work, but even then it would be a pain to find people to do old scenarios.