Returning from 2+ year break

[MiRai]

Welcome back.

1. Is /follow still disabled for PvP? If so, how are people dealing with PvP battlegrounds?

To answer your question, as it's written, is: No, follow is not disabled for PvP, but it is disabled in random/rated battlegrounds, war games, and Ashran's open-world PvP zone.

2. Is Jamba still around? Or what main MB addon is recommended?

JAMBA is still around—it's actively discussed and has two stickied threads in the WoW Macro & Addons sub-forum.

3. Have they changed the macroing system in important ways for multiboxers?

I'm going to say no, but I don't know what you remember about all of it.

4. Any links to game setup info? ... I recall having scripts to hardlink/symlink certain files to 5 "working" directories for efficiency and separation of accounts.

With a few different choices of software to use, you'd have to be more specific. I would assume that the vast majority of multiboxers today use ISBoxer, so symlinks are completely unnecessary unless you have a very specific reason for using them.

5. Any other general recommendations? I left before Garrisons, etc. Will need guides/walkthroughs for leveling, etc.

I don't personally know of any guides since leveling is the same as it's been for awhile now, but perhaps someone else knows something useful. Other than that, I'm pretty terrible with general recommendations, but as you keep thinking of questions, just ask them and people can answer.