Would make it very easy to level new characters by taking 4 max leveled chars into a dungeon and just feeding tons of exp to the new char. I'm guessing that's what they wanted to avoid. You can still do that but the new char would get their normal share of exp instead of 5 times the amount.
Aerith does have some niche uses for her SB which is AoE Stop + Silence. The Oilboyles in one of the elite levels and the three sisters in another ones are good examples.I powered through the regular side of the event and got Aerith, but I doubt I'll make use of her much, not when I already have a level-broken Selphie for healing duties.
Jenova is supposed to be a fair bit easier this time than last (I guess since it's not the last boss). I would definitely push to get Faith from one of the later fights and then do the Ex+ fight (or ++) for Diaga. I think it should be just a first time reward and not a mastery one so it should be doable.I'm doing the Elite run that rewards a third Mythril Staff so I can combine it up to 4 stars, but other than that and powering through Jenova to get Aerith I'll probably ignore it and the Relic Draw.
Selphie or Aerth have the highest Mnd at 65. Lenna has the highest at 50. Lenna's default SB is also nice as a mass regen. The fact Lenna can't use an RM is a big downside though. Being able to throw R1 Protectga up every fight and still have 2 up for the boss fight is very nice (with WM RM 2 of course). Tyro can work too if you need a different ability and can't fit it in the rest of your group. He's good for holding one Curaga and then one niche ability, especially those that don't need good stats to work (intimidate, the busters, even breakdowns if necessary).