What's the basic plot outline for the party member social links?
Are the randomly generated dungeons in Mementos better or worse that P3/P4's approach?
Which character do you think will be hardest to portray in the dub?
Given how late Haru show up, does she get enough screen time to really distinguish herself from the rest of the cast?
How obnoxious are the stat requirements for social links and how difficult it it to gain social stats?
They vary significantly, but most have at least one personal stat check. (Haru's is particularly brutal, since you can't even get to rank 2 - and unlock the extremely valuable Baton Touch skill - without maxed technique stat.) I can give examples of plots but I don't know how much detail you would want.
I didn't feel there was that much difference between Mementos and Tartarus? Mementos doesn't have any set midbosses/bosses unless you accept sidequests, so it's pretty easy to blast through a block if you're so inclined. Never played P4 so I can't compare to that.
Hardest dub is probably Futaba, Morgana, or Goro.
It was close but I think the answer is yes. Despite joining relatively late in the game there's still a lot of plot and dungeon left before the end, so she gets a good amount of screentime. They also wisely made her co-op (SL) available almost every day so you could catch up quickly.
I didn't have a terrible time gaining stats or doing co-ops (I forgot to mention I maxed all the co-ops without a guide), but you do need to eventually max all your personal stats to finish all the co-ops. Several co-ops give you a bonus to a stat for most of their rank up events, and a few co-ops have skills that help you manage time more efficiently (e.g. "pay to increase affection with someone once per day", "have this person do a time-consuming activity instead of doing it yourself".) That said if you're shooting for max stats/co-ops you will need to be proactive about gaining stats. IIRC courage was the last one I maxed.
What difficulty did you play on, and what did you think of the overall balance?
I played on Normal for most of the game, but I switched to Hard about halfway through the second-last dungeon. After a bit of a dodgy start things leveled out and got pretty relaxed for most of the game, which is why I bumped it up to hard. Disclaimer: I did use the EXP-boosting charms from the P3 DLC every now and then, but I was apparently 10 levels lower than the average when I fought the final boss (who felt very manageable), so I don't think that threw me off that much balance-wise.