jubileee, I'm a hoarder. I can't speak for all of us but I can share a little bit of my experience with this aspect of Obsessive-Compulsive Disorder.
Currently I'm 37 and have been in treatment for about 5 years for OCD, depression and anxiety, all of which feed on one another. I was headed toward being like your future M-I-L.

A combination of therapy, drugs and an understanding family have me at the point where I can throw things away. It is an enormous step but probably wouldn't look it when viewed from the outside. I still have a giant hoard of items that needs to be... I suppose "processed" could be the word. Keep, toss or donate? It is so difficult to determine that it can be paralyzing.

I know all of your MIL's items seem like junk to you. But when you're a hoarder, things as silly as receipts and magazines can acquire an extremely high value. I'm using those as examples because you should take note of how attached one can get to even the most trivial of objects.

I don't know your MIL so I can't speak for her, but I have every reason to suspect that she cares deeply about all of her things. That she has storage units for this stuff speaks volumes.

This is why you have to tread carefully. The collection she has amassed over time is significant in size and in emotional baggage.

Unfortunately, it is extremely hard to get hoarders on the path to success. I believe you have to really want it, and since she's lived like this for so long, she's not seeing how damaging to herself and her loved ones this is.

I wish I could properly articulate how it feels on the inside.

That being said, she needs psychological help, no question. Introducing the idea of getting rid of any of her stuff will be extremely stressful. The personal organizer angle is a good way to go if you can afford it.
Your future husband needs help, too. I don't know if he's aware that he's enabling her, but he is and you really need to talk with him about it.

The two of you should read as much as you can on the subject. And I suggest watching episodes of the TV shows. You're already taking steps in the right direction by coming here and asking for help. (Don't forget about your own feelings amidst all this madness.)

It won't be easy. Not one bit of it. But if it means healthier, happier loved ones, it's worth it.

I hope this is somewhat helpful.
~ angelene