In general:
If I find a game-breaking mechanic that I can use, I'd inform the mod and see if he thinks it wise to change the rules in this case (ideally, with a concretenew rule/restriction like changing my ability to n-shot or preventing me from using it on a particular player.)

In the specific example you gave, that's just terrible. If it was me I'd immediately inform the mod before the game started and just ask him to change the setup. Unless by some miracle he can find a way to change the rules to nullify the game-breaking advantage, but I can't see this possible. Note that: "new rule: you can't tell others who the mafia are" is not a concrete rule and extremely hard to implement. Where do I draw the line? As a player I am expected to make reads, so what reads can I do against them? What I find them real scummy? But how can I tell if I do find them scummy or am just saying so because I know they are.

As town I'd feel not happy either way. I won, but did I really win if my opponent had by himself to put aside a winning strategy?

Also note that putting restrictions on yourself, in general, is a bad idea because unlike the mod you don't know the full setup. You might be thinking you're being a good sport nullifying an unfair advantage that you have, but in reality the other faction might already be somehow compensated for it, and you're in effect under-powering your own team.
Pretty much, yes.
Scenario:

Tragic town lover: you win if You and Mafia lover survive OR if town wins normally (all mafia are eliminated - in which case your partner loses)

Tragic mafia lover: you win if you and Town lover survive OR if mafia wins normally (they obtain lynch control - in which case your partner loses).

(Note that the common win condition means that both regular town and regular mafia lose).

You are mafia lover, and you agree to go for the co-op win only. Yet as soon as the situation becomes TTTLL, he decides it's too much trouble to go for it, and just outs you and wins. You feel betrayed and hold it against him.

But if you were one of the T, don't you think that there was an implied agreement between all Ts (including the lover) to go for the Town win? Would you feel betrayed that L(t) went for the co-op win condition causing you to lose? Didn't your implied agreement essentially mean he betrayed you?

EDIT:
OK, I read your post and in the latter case you'd hold it against the mod for being bastardish. But the lover did have the same condition as you in the above scenario. And he decided to go for the other one.