What a neat thing to find!
Sorry about your loss!

There is a product called Archival Mist. You can usually buy it at scrapbook stores. it is not visible when you spray it on paper. It simply stops the aging and yelloing process. I used it on some newspaper clipping from WW2 of my grandpa's. It is what museums use to preserve paper.

As fat as what to do with them, I would put them in an acidfree binder with clear plastic acid free sheets (no glue), maybe one valentine per sheet. That should preserve them, and make it so you can still see both sides. Don't glue them to anything!

Maybe put a picture of the previous owner in a sleeve along with a little information about her. I think that this is what a museum would do.

Along the lines of the collages, some people combine old cards with pieces of vintage quilts and bits of memorabilia from the person. You could do a bunch of these and frame behind glass to give as gifts to family members of the deseased love one.