These are some of the very same questions I have been wondering about ever since I heard about the 300+ achievements coming in 1.20.

Now, my preference is not to have achievements awarded retroactively. But I think that largely depends on what type of achievements they've set up. Will they be similar to the player history on Lodestone, just a bunch of milestones? "Reach level 35 on Pugilist", "Discover all 24 camps", "Complete 100 local leves". I can never level Pugilist back to 35, so if that's one type of achievement they've added, they'll have to be unlocked right away. And it's not like I can rediscover the camps or undo the local leves.

Hopefully the achievements will it be something a little less mundane and a bit more fun, "Defeat an NM naked" or some such thing.

If I recall correctly, the in-game achievement and the Lodestone player history are going to remain separate. I thought a Dev or Yoshi-P said that somewhere, but I read a lot of stuff on these forums and just might be remembering a statement someone made that fits with what I think they should do.