Your editor doesn't seem very supportive, and if you're able, I would try to find a new one (probably a member of the LGBTQ community, if possible) who gets your vision, or at least have a serious talk with them about their holdups.

I think one of the things that is getting lost in translation here is that both sexual attraction and romantic attraction are distinct spectrums and don't always "align". Some people are bisexual but heteroromantic, some people are heterosexual but biromantic. I have a lesbian friend who is very capable of falling in love with men (she's biromantic) but is only attracted to women sexually, and that, unfortunately, led to a very similar situation to yours where she married a man that she loved but ultimately they weren't able to make it work because the attraction wasn't there. It was very sad because she did love him and they did try, but a person can't control their sexuality. We lost a few friends during that misadventure to the same kind of prejudice you and your ex-wife faced, with people coming out of the woodwork to say bisexuality wasn't real because she came out as a lesbian. It's very frustrating, but from your comment, you seem to have a really grounded understanding of the situation, which I must applaud.

Back to the writing bit: a person definitely can be both bi and demi, or bi and ace. The asexuality spectrum (from allosexual to asexual, with demi in between) only dictates how much attraction is happening or how it forms, not to whom! Being both bisexual and demisexual is totally possible, and your editor is just not understanding that.

How your character identifies definitely shouldn't be up to your editor. You know your character best, and if they're both demi and bi, that's who they are, period. Your story sounds really interesting and I hope you're able to stand up for it!