The legendary quest I have is telling me to go to an area in chapter 4; should I just re-roll it to get terminal/MM?
I would say never do this. It's a huge credit sink and it's usually not THAT much more time to complete those other area objectives. If you're really desperate for odin marks fast then, meh, go ahead... but I wouldn't recommend it.
I turned on synergies for 9 characters above level 30 but I don't see the XP bonus lit up; it's still at 0?
Synergies and XP bonus are 2 different things. IIRC, it should show on your stat screen somewhere but I can't remember if it was really level 30 that gave the first xp bonus or not. I thought it was 40 but *shrug*.
How are Loki, Jean, and Iron Man currently? My IM is 55 and I got to play him a bit after he got the One Off for the first time but the leveling was pretty miserable (which was part of the reason i stopped playing).
My Loki is only level 23 and he sucks so far. Lots of his better skills seem to be 30+ in level and he's got spirit issues like woah. I think I would of been better off playing summoner for the early levels then switch to caster-mode but meh. He's garbage at the moment but that'll likely turn around later.
Jean I own but have yet to play.
Ironman is pretty beast and an easy leveler IMO. 0 spirit problems and half of his best abilities are early in level. Wrist Rocket/Pulse Bolt are amazing AoE's and barrage is good single target. He was an incredibly easy leveler for me... *shrug*
What's the fastest way to level?
MM till ~10 or so, then X-defense till 20, then legendary quests till blue in the face.
Still don't understand Relics. Do I really have to collect multiples of the exact same one to see increased benefits? Seems pretty hard to get anything going given RNG?
Yes, and for the most part, they hit diminishing returns hard and fast. For the most part, getting up to about ~100 stack should suffice, the difference between 100 and 1000 (other than 900 HP) is generally pretty small. The only real exceptions are the +HP one (I think that's just 2K HP at max stack), +defense one, and +spirit one. The rest you can split if you really care.
Are Rings, Uru-Forged Slot, and Legendary just more types of gear that can drop like any other piece of gear?
Rings and Uru-Forge drop. Legendaries you buy with Odin marks only (for 300). Runes drop that can be socketed in slots 1-5 for small bonuses (see your enchanter) or combined in a Uru-forge to form a runeword. 3 to look out for:
River of Soul (Spirit) - Chalice, Lake, Man
Ancient Grove (Defense) - Ash, Birch, Oak
Seer's Brilliance (Leveling) - Lake, Year
All 3 are relatively "cheap" to put together and offer some amazing bonuses. Not sure what level enchanter you need for all 3 however.