If you DO believe but shouldn’t, is that enough to be a firsthand witness?
No, I wouldn’t think so.
I think you (and I) had originally thought the issue (or one of the issues) was whether or not you’re allowed to be mekabel something even if it’s not l’toeles at the time but it becomes l’toeles later on. But I realized afterwards that you’re never allowed to be “mekabel” even if it’s l’toeles, so it seems like it would be a problem in any case.
Another problem is that it only became a toeles AFTER (and as a result of) you were mekabel. So you can’t say you were allowed to be mekabel for the sake of toeles.
Does that make sense to you?