Ok so you have no evidence he's relapsed but according to previous behaviour your suspicious? Past behaviour can be an indicator of future behaviour. Ask him outright about the fines, you have the proof that he didn't pay the amount he told you, there may be a simple explanation or he may lie either way you will know if he lies.
Talk to your friends I know you fear they will judge you but maybe they won't maybe they will listen and offer support. Do you have a counsellor or attend any support groups?
I understand the thinking and driving yourself mad I do it all the time but it doesn't help just makes me feel worse as it increases my anxiety trying to figure everything out. Bottom line I can only make decisions that are good for me based on what I know and feel, but it's hard to know when we are caught up in the cycle of their addiction. Our focus is on them not us and we don't know how we feel about anything except how to make sure they're ok, if they're ok so are we. But we are not we are just waiting for the next storm to pass through and pick up the pieces again when it's calmed.
Go for a walk don't think about him think about your feelings, what do you want for your life, talk to your friends.
Dandylion often advises to use the wailing wall, go somewhere quiet with no one around, scream, shout cry until you can't shout anymore say everything you need to say don't think about it just let it all out until the tears no longer flow, the anger is gone and you have nothing left to say. You will be amazed at what comes out and you may even learn something about how you actually feel. It works I have used it so many times.