- 2 additional expansion since AI: Lost Eden and Legacy of Xan. On top of that, several end game instances/quests. And ofc, each expansion brought in more powerfull items/nanos, etc.
- Yes, the curve is still there. In a game, where you can have a setup of your liking, this won't change.
- No idea what you would call active PvP enviroment. Escpecially since I'm not into PvP. But you have battlestations (speciall instances dedicated to PvP), that run 24/7, at least at certain levels. And there will be additional PvP instance added in next patch.
- there is active PvE enviroment. Most is done within orgs or patched up teams, but there are several done by public bots still.
- No idea which class is best at end game solo PvP
- Raid interface allow 36 players (6 teams of 6 players). But only couple of instances allow more than 1 team (Pandemonium, AI sectors, 12man). Still, where it's allowed, players take advantage of it.
FC also added several options to gain xp/sk easier. Still, I'd estimate you'd need 4 months to level MP to 220/70/30, get the best gear and le-learn the ropes. I suggest you go to Search Guides
page and check guides added after patch 15.6.1. (Alien Invasion). That way you'll be able to judge what changes were done since then.