Hi!! Sorry this took so long to answer; I was gonna do a little sketch of the genderbent Underwoods to accompany my answer, but with holiday hours keeping my nose to the grindstone I've regrettably given up on that. Maybe some other time!

You're right that the Underwoods would all differ drastically in a genderbent AU! For simplicity's sake I'm going with cis genderbends, and assuming events still proceed more or less as they do in canon - that f!Alec still kills her mother and gets sent to Indonesia for it, for instance, and that things proceed the same with Morsden and Bertrand and everything. f!Alec would, I think, not be too different from canon Alec because of the way neither of them really buy into gender roles. Internally she'd be much the same. However, because of the way women are expected to behave in 19XX English high society, she'd present as much more gentle, humble, and self-effacing, and probably act primarily by steering the actions of her husband, m!Viola.

m!Viola would have more marked differences from canon than f!Alec. For one thing, he'd never have been a ballet dancer, and would have instead wanted to inherit the family business and restore the Crane family to its former glory. Being raised with that aim - not to mention being a manlet with a domineering, older wife who babytrapped him when he was eighteen - would see him with a chip on his shoulder, and he'd be more aggressive and likely to act out of anger/other emotions than Viola is canonically. Which leads me into some of the differences with m!Isabelle!

For clarity, m!Isabelle is still genderfluid, just AMAB instead of AFAB. One big difference would be that f!Alec would seek to raise him to be a proper heir. While she would still be psychologically, physically, and sexually abusive, she would keep this more under wraps, never crippling him and certainly never letting on to m!Viola that she's fucking their son - primarily because m!Viola would take this as being cuckolded by m!Isabelle, and would probably try to kill him. m!Isabelle would be raised with more freedom and better education, and would be encouraged into his parents' image rather than forced into the tiny pigeonhole that Isabelle gets in canon. As a result his compassionate tendencies would be more buried, and he'd be something of a tsundere about them, while also being in denial about the fact that f!Alec is abusing him at all. He'd like to think of it all as either consensual activity or just proper child-rearing. Nightmarish, I know.

If you have any specific questions about any of them, feel free to send them in! This is getting long.