] Could it have been as simple as the Knights didn't want any children, but they wanted to have an heir? Adopting Edward was certainly less troublesome than having kids of their own.

I had the same thought. He was 44,rather old for marrying the first time, but not too old. And she I think was 26.
I also thought that maybe his health was failing and they didn't want to take any risk if he should die early. But then I read that he died suddenly of a stroke, so that theory doesn't seem to hold. My best theory is that he may have been impotent.
Yes, when I think about it, doesn't that sound plausile?
And that can explain why he waited till he was 44 to marry.
He may have told her about it, and she may have accepted marriage anyway. She may even have felt relief.
And that's why they pre-planned the adoption.

Yes I'll go for this theory. Disprove me if you can!