If your fiance is okay with it just tell them to pay their own loans off. That's terrible of them to act like the would pay it then stick it on you two. Certainly don't get into any situation where you are legally transferring the debt from them to you. Even if you get suckered into starting to pay it off DO NOT transfer it to yourselves legally... then there will be no going back.

Is your fiance okay with putting the loans (that they agreed to pay) on them, dis-inviting them from your wedding, and never talking to them again? That would be my preferred course of action. They don't sound worth putting yourselves through anything for. You don't even have to say "Screw you, pay your own stupid loans" to them - you just have to calmly explain that they signed on to and agreed to cover those loans and you're not going to take on that responsibility as it's not yours. Are they just mad at your fiance for being with you so now they are trying to back out of paying?! Don't let them get away with it. You are in the position of power and also you're in the right. Don't let them guilt you into taking on their responsibilities.

As far as work, just keep busy and work steadily just like you're doing now. That's really all that is expected of you. Someone is getting rich off of your work and it's not you so don't kill yourself for their sake. You just need to do your job and work steadily - you don't need to feel guilty for that. In fact working steadily and doing your job is something to be proud of... killing yourself at work to make a couple extra dolalrs for someone else is a fool's errand. Don't slack off but don't work yourself into an early grave for someone else's benefit.