Where can I find a diagram for threading my montgomery ward sewing machine?

Unfortunately you have uncovered the problems with slab houses. There is little you can do short of cutting the slab and rerouting piping and venting. Due to toilet piping size you may want to leave that in places. You could use a macerating toilet but they are pricey. You may be able to tun the smaller lines under or behind a vanity and the use an AAV if code permitted in your area. Lots of heavy work.

I do not understand the problem. You put the thread through the needle and sew. It's very simple.