I'm late to the party, but that's definitely an 1881 Marlin. If you decide to shoot it, I would have headspace checked along with its general condition. There are no locking lugs, the only thing holding the bolt closed is a cam surface on the lever. Keep loads at trapdoor levels.
The rear...
Maybe so. Everyone I know doing retail said this whole year has been slow. Custom work is still thriving. I just let people tell me what they want instead.
Drill and tap a hole perpendicular to the windage screw that would allow a set screw to be tightened against the diopter body. You can line up the new hole with the 0 on the windage scale, but come from the backside if there is enough material there.
1/2-28 is fine for 7mm.
Some suppressor manufacturers site possible side load weakness if the rifle is dropped on the suppressor, on the smaller wall thickness for their reason to recommend larger threads.