I think I would wash first - probably by hand. If they fall apart, you would probably want to know now.
As for setting them together, here is an idea that I used for prequilted blocks that worked pretty well:
Decide which side you want a little strip to show on. If you want it decorative with a contrast, make it show on the front. If you want simply to put the blocks together next to each other, put it on the back with a harmonizing color.
Next cut strips about 1 1/2 inches wide. You could use packaged wide bias tape if you have a source. Then put the blocks together (we will say back to back), with the side of the strip included in the seam allowance. Sew it. Then open up the blocks. Fold the strip of fabric over the seam and either hand stitch, or top stitch. It covers the seam and makes a little sashing between the blocks.
Sew each row, finishing the strip covering the seam. Then, sew each row together, putting back to back and a long piece of 1 1/2 inch wide fabric included in the seam. Then fold it over the raw edges, and you have a completed top.
Is this clear as mud? Hope you (or someone) can use this method sometime.