Thinking of the case of a wife or husband of ten years who was in shape and is now pretty fat (like when insulin resistance is finally pushed past the limit as can happen).
If the other spouse is still svelte they do not have to say that their fat spouse is actually thin, but also do not have to point out he/she is fat all the time. But if asked by their spouse "am I fat" then "yes" is an acceptable answer - preferably followed with other positive praise. The question is how much positive ledger does the rest of the relationship have.
Is the child putting a lot of eggs in the basket gender identity out of a sense of insecurity?
If dad was like "I am skeptical that you are transgender and will not desist, but I accept YOU come what may" is this an acceptable communication to the child?
No. Effectively, you are telling the child that you don't believe them when they say what/who they are....followed up by saying you accept them, come what may. Really Obviously, you don't accept them as transgender. You've just lied to them. Why should the child believe that you 'accept them' when clearly you don't? And what does 'come what may' mean in this context?
It is acceptable to say to the child that you're struggling with coming to this new understanding of who your child is, that is is your responsibility and to apologize if you sometimes make mistakes. And then you do your very, very best to not make hurtful mistakes. AND you get yourself to some good counseling so that you can fully understand and accept your child as who they truly are.
If this is difficult for you to understand, try this: Would it be acceptable for a parent to say to their child who has just come out as gay that the parent is skeptical that the child is actually gay but that you accept the child, 'come what may' whatever the hell that phrase is supposed to mean in this context.
Because, putting on my 14 year old hat, that just sounds like: I don't believe you but I love you even though you are confused and delusional and I'm sure that someday you will fully accept my version of who you are as your authentic self and then I will truly love you because you reflect my beliefs when you are who I say you are.
I think it is perfectly acceptable to acknowledge that shifting your perception of this person is going to take some time and adjustment on your part, that the hard work is all yours and not the child's and that you apologize in advance if you say or do anything hurtful, that you hope the child will point out to you if you do something that is hurtful and that you will do your best. And then YOU DO YOUR BEST, even if it feels wrong to you because your child's wellbeing is on the line.