Questions I have about the game;
1 - I'm looking for suggestion on gear that's doable to get either through quests and/or a single group. What kind of gear should I be going for past the elegant/elaborate defiant in my mid 80s to early 90s with that in mind?
Abstruce gear shouldn't be to hard to obtain, and is quite nice.
2 - Weapons for the off hand. Should I stick to a piercer or take any other 1Handed? Any suggestions for mid 80's to early 90s?
The best ratio you can get is the way to go for now. At 95 with aa's you will eventually switch to only piercer.
3 - AA's. I'm going through the damage avoidance ones first. Then the proc and proc crits. After that I'm at a loss and I'm being drawn to Anatomy and maxing out Hasty Escape. I currently have about 75 AAs invested. What are some recommendations for the next 800 or so?
Depends on your playstyle, it sounds like you have a number of friends still playing. So if you find yourself grouping most of the time, I would suggest getting the first 3 ranks of combat agility and combat stability but other then that go primarily offensive aa's.
If you find yourself soloing for the most part, then I suggest keeping anatomy maxxed and mix up your defensive and offensive aa's as you go.
At the bottom of that page is a good list of places to go where you will be able to assassinate until 90.
4 - I'd really like to experience some of the content from the following expansions, either molo or in a group. The Buried Sea, Secrets of Faydwer, Seeds of Destruction, Underfoot, House of Thule. Any suggestions?
Seeds and previous you can prolly molo most of the progression, the other, bring your friends! Some pretty fun missions and stuff in there, definitively worth going through
Also if you don't know about them already utility poison's are nice to use as well. In both group and solo situations. You can use one of the utility poisons along with the DD poisons you buy in pok. The table in the link below will help you with making them, and tells what they each do.