I'm not sure if I'm interpreting your comment correctly but, whether or not someone is able to pretend they love someone irl, asking for "proof" is still insane. Even more so in real life than in the extremely specific context of this manga.
When all you've ever seen from someone is devotion and demonstrations of love and affection, whether or not they're internally cursing at you is irrelevant. You can only judge people based on what they act and speak on; their innermost thoughts will always be a mistery. You will never know for sure what *anyone* actually, really thinks – so you could, in theory, make suppositions about anyone thinking anything.
That's why, it'd be stupid to distrust what someone is showing/telling you when you have no tangible reason for that distrust. When they are baseless, these suppositions and what-if scenarios mean absolutely nothing.
So if you can't trust someone because they *could secretly hate you,* don't. Don't trust them, and move on. Just don't ask for "proof". It'd be ridiculous to ask for that-- you are just admitting you don't trust this person. If they are showering you with love because they have genuine feelings for you, and you don't believe them, then their love simply isn't enough for you.
So, what more could you want? If someone already gave you everything they should've, what else could you possibly ask of them for them to be enough? How much will you take until you're satisfied? And how would that be fair?

I legit feel so bad for Theodore rn bc he's given her NO ammount of reasons not to trust that he loves her – in fact, he's been on pretty much perfect behavior 'till now – and now he's being asked to PROVE IT? Wtf does she expect him to do
She can choose to believe him or not, idc, she has her reasons either way-- but saying "prove it" when he's actually been nothing but sweet and loyal??? What