OK. I have no idea if your DP is cheating now, has cheated in the past or plans to cheat in the future.
You have no idea either.
DP is not ever going to tell you.
You have a 'feeling' and some evidence, but it is not enough for you to confront/leave.
So, I think in your case, you need to take a different approach. For some, his 'previous' and the current condom would be enough to instigate an almighty confrontation that ended who knows where, but you have repeatedly said that you are not prepared to that in case you end up with your 4th charge of false allegation. Again, neither I nor anyone else knows if that is the case.
OP. You cannot stay in this limbo but you are not going to confront without evidence, so in your case, I would suggest that you do whatever it is you have to do to find out one way or the other:
I wouldn't normally suggest it but check his texts regularly - you will know how many have to 'mount up' before he is obliged to delete them. If they are being deleted sooner than is necessary, this is evidence of hiding something, no?
Get someone to call that number. Find out whose it is, how old they are, what their relationship status is etc.
Put little dots on all the condoms in the house - number them if you have to, then you can know if any 'disappear'
Get a private detective on it if you have to.
Me, I would go on instinct, but I DO understand that some people need black and white evidence, so get it.
And when you have your answer, you can act accordingly.