
Why would he fake any of it when he doesn't have to be nice to her though? She is a destitute nobody. He didn't have to marry her. He could have have kept her imprisoned in the manor somewhere. He could have sent her siblings off to a boarding school or an orphanage. He could treat her like complete garbage and nobody would do anything or think the slightest thing about it.

My dear, please think more critically... Through histories of anecdotal experience, it's obvious that it's far easier, and smarter to keep people by your side by being nice rather than being cruel. He needs her to be by his side willingly.
What do you think would happen if he treats her poorly? She would have to rebel, or worst, take her own life. That's not beneficial to either of them. Either way I don't know the ending of this story or his intentions, but he SUSPICIOUS as heck is what I'm saying, and the FL has every right to feel suspicious too.

Not hitting, disrespecting, kidnapping or being otherwise abusive to other people is the bare minimum of being a decent human being. He is not a good spouse because he didn’t resort to those means. Also, considering from her knowledge he does end up murdering her in another life, it would be stupid for her to let her guard down and not have an escape plan.
He's offering you anything and everything a husband possibly could free of his own will. You're the one allowing disrespect. You're the one acting like your marriage isn't real. He's done nothing but be supportive in every way possible