I may not be much help on that negative holder but this is what I think from the pictures:

The negatives go between the pegs. The mechanism holds it open to slide the negative to the next image. It should be closed tight for enlarging.

Your 50mm enlarging lens probably has a 25mm thread with a 39mm adapter. I suspect you need a 80mm enlarging lens with a 39mm thread. However if you cobble the adapter from the other lens you can likely get a very cheap six-element Componon 80mm in the 25mm mount that no one wants. I got one for five bucks once.

The adjustable light bulb is to fine tune the evenness of illumination.

That easel looks fine.

You can start printing with the 50mm lens if you do a severe crop of just the center of some of your 645 negatives. Just to see how it works as you are looking for an 80mm lens.