Yes those lights will plug right in. You may have to space your taillight brackets a little from the frame for clearance for them. Depending on your harness you may not have a blackout stoplight wire for the driver's side (not a big deal for 99% of owners, probably 100% on this forum).
The valve stem is loose from the rim because the military tires use inner tubes. If you're going to run tires tubeless you'll need to pick up a couple valve stems from an auto parts store and put those in the wheels. I've run tubeless tires on these wheels before with no problems.
The older trailers only have two crossmembers, while the newer ones had three. The are differences in how the tub mounts to the frame as well. The one I had with two crossmembers used 3 bolts at the top of each fender (most use 1), and it had 4 tabs on each side holding the tub to the frame...
All m101 trailers had parking brakes. The bolt pattern is not an a1 pattern, the axle has been changed at some point. The frame also looks to be heavily modified. The a-frame on an m101 series trailer sits underneath the frame, on this one it is level with the frame and welded. Also there...
Got pics of the wheels? Every m100 I've seen doesn't use "split rims" or lock ring style wheels. If your wheels are stock you should be able to run tubeless tires fine, as long as they aren't leaking around the rivets.
The hubs on an a1 are commercial 3500lb with regular bearings so it should be easy to change them for a different bolt pattern. There is a post on a forum that I can't remember where someone has done it but hasn't posted pics.
The surge assembly is unique to the m416a1, but the internal parts and lunette itself are the same as m101a2s and a3s and are easily obtained. Its worth something to somebody, especially since the tongue is often removed or altered (in an way that can't be fixed unlike yours).