Hello, return!
1) Impossible to say for sure, at peak times in EU I would guess around 1000-1500 on Crom. Other servers are dead.
2) Crom is the only server with a healthy population. Even hardcore PvP'ers come to Crom to have more minigames.
3) Server merge: yes. Eu and Us are now merged into one server. There are three servers. Crom is PvE, Fury is PvP and Rage is Blood and Gory.
4) Funcom has had halloween events going every year since launch, so I feel certain something will be happening this year as well. There are also talks about a subscription offer in November.
Otherwise, Tony82 has it summarized quite well what is new in game the last few years.
As long as you are aware of the bugs in the game, and how to avoid them, the game is quite enjoyable. There are many friendly veterans around in game, and if you get into the "right" guild, you will probably not regret it and have a kick ass time with Age of Conan again!
You might find this thread helpful if you are struggling with some bugs; http://forums.ageofconan.com/showthr...he-Game-Errors
And also if you get some crashes, check if you have these windows updates and if you do, uninstall them; KB2670838, KB2484033, KB2488113, KB2506928, KB2511250, KB2522422, KB2545698, KB2547666, KB2786081 and KB2799926. Some of these are part of Internet Explorer 10/11 and you can not have that browser installed if you wish to have a crash-free experience in Age of Conan.