I don't think you should discount yourself. This is already affecting you and you are not yet married. I think it is wonderful that you are so willing to understand her illness and to help!

Now, getting your future husband on board is crucial. This problem is snowballing and will continue to do so if left unchecked.

It sounds like you are trying your hardest and that is commendable as others might have run in the other direction after learning about your MIL's condition.

Digitizing things is a grand idea, especially with the tapes. If she's uncomfortable letting other people handle them, there are DVD/VCR combos she could use on her own or with help...
Of course you have to convince her to do all of this, which is a huge hurdle.

I wish I had better advice to give. I really do hope you can get through this with your future husband.