Duma isn't great fodder imo because Bold Fighter is on a billion other, less premium units and Def/Res solo is pretty niche when other others like DC or breaths are better in the A-slot for EP units.
Duma I feel like is a bit like Meisterhardt in that he's a rare unit where the skills you can get from him are on other common units. It's a bit of stretch with Duma, though, since Bold Fighter while on a lot of units, is a 5*-only summonable skill while Meisterhardt's Death Blow, Vantage, and Pavise if you really want that are on a 3* to 4* summonable units. Spur Atk/Def is the only 5*-only thing he has and shared with summer Tana, but anyway, unless you desperately need one of the skills Meisterhardt has or really need to get rid of units, merging him or fielding multiple copies of him would likely go a long way. Same deal with Duma who's also a mythic hero, so he can provide stat bonuses and Upheaval stacks as you noted.
Duma's a very stupid unit when you think about it even if some people think the mode he was being sold for, Aether Raids, is where he's actually not that good. He ties for the second highest attack at 40 base neutral with winter Chrom, Effie, legendary Hector, and M!Grima and like legendary Hector and M!Grima, he has an attack superboon bringing him up to 44 base neutral. The difference is that his personal breath takes it further with its +3 Atk stat boost and enemies who aren't at full health add another +6 to his attack and resistance. That's 63 total attack against full health foes and 69 against units who aren't at full health. Against some units, Duma doesn't even need Bold Fighter or Upheaval damage when he can straight up one-shot them. M!Grima can do the same thing, especially against blues, but his raw attack power is lower than Duma's. His downfalls is that he is still an armor and one who doesn't come with Distant Counter in any form like Caineghis, M!Grima, or the Black Knight/Zelgius, so he can be cheesed by ranged units. Being an armor is also a problem for a unit who can one-shot units since it's going to be very difficult for him to get to them without some help. His defense isn't also that great. 34 base is still high, but it's not stupid high like Surtr's 40. And Duma being a colorless unit is a double-edged sword where he isn't weak to any units, but -raven mages, anti-dragon units, and anti-armor units and he's also not strong against anyone else either.
I would say that Catria's -1 special cooldown is better than Est's armor effectiveness, but both are very good obvs–even Palla's sword is good, even if TA isn't as broken as the other two.
Catria's Whitewing Lance is the most flexible of the Whitewing weapons and her stats are arguably better distributed offensively since she's not like Est who dumped some speed to have high attack or Palla who's a jack of all trades and the more defensive of the sisters. Est's Whitewing Spear, however, and her high attack means she can tear through armors and sometimes regardless WTD. In general, she will hit the hardest too with her much higher base attack than her older sisters. People harp on Palla's Whitewing Sword being a personal Ruby Sword, but I feel like it was deliberately chosen because of who Palla is. This would also go with why her default weapon was Ruby Sword to begin with. Palla always looked after Catria and Est and in gameplay for FEH, that means she's covering for their weaknesses which in this case would be greens and in an extreme way as well. At the same time, I get the feeling she neglects herself in favor of her sisters and in-game, that means she's leaving herself even weaker against blue units. You could even argue that Palla being that weak to blues is if she ever found herself in a situation where she had to fight Catria or Est, she would definitely hold back to prevent hurting them.
The developers probably just gave them personal weapons with their default, generic weapon's base effect, so I'm probably just overthinking things, but it just makes sense to me why Palla's Whitewing Sword would be a gem weapon and why her role as an anti-green specialist makes sense when you think about her sisters who are both lance fliers.
Florina's prf is kinda disappointing because it didn't really do that much to fix her glaring flaws. It seems IS looked at her awful statline and were like "uhhh....turn her into a weird armorslayer/melee tank." Florina needed the Niles treatment and IS slammed the door in her face.
Well, they were the ones who gave her those stats when it made no sense for her considering what she was like in Blazing Blade. Florina stats probably should have been closer to Caeda's which would probably cause a problem with Clair, Shanna with less defense and maybe health for resistance and speed, or even Selkie's which would be hilarious since she'd be so damn frail and without anyone really to bump up her defense like Selkie can with her Kitsune Kit Fang.
She can still end up being deceptively bulky, especially physically, and fast with her lance. It's just Florina is a weird and unfortunate creature.
Should I give DC + Null C Disrupt to Donnel? I've decided that he's going to be my next merge project. Also, is the res superboon worth considering on him? I feel like +atk is generally more useful because it affects all of his MUs, which res is only relevant vs. mages/manaketes/healers.
Do you trust in him being able to reliably one shot or survive enough encounters with +Atk over +Res or do you want to maximize his resistance as much as possible? There's also +Spd if you want him to avoid as much doubles as possible and ensure more doubles as possible.