I doubt the loctite will work here; high temperatures degrade it. Perhaps the machinist is knurling the seat so it'll stay put.
I've been following this with some interest; it reminds me of the guy on one of the Ural boards who went through 4 engines; two outright replacements, and two re-builds. Some was surely random chance (older Urals are examples of Russian "just good enough" philosophy; crappy materials and shoddy workmanship, but a design robust enough to tolerate it. For that matter, so are newer ones, but they cost more), and some may have been helped along by a mismatch between perception and reality. Once he got good at repairs, the problems needing repair went away, I think because he started understanding the machine.
There's a whole list of suggestions here, none of which you have really responded to--see Foggy's post earlier. High performance parts aren't going to fix basic problems, and you have basic problems.
Have you checked the jetting by doing a clean ignition cut under full load? I would expect that you'd be running at least a 120 main jet, probably richer; better look. Lean jetting makes heat, heat kills engines. That's happening to you.
Are you dead sure about the valve lash, secure adjusting nuts, etc? A little loose is likely OK; a little tight, and it probably won't start, but check it, it's easy.
Are you dead sure about the timing? advanced timing makes heat, melts pistons.
These three things are fundamental. Unless the mixture, timing, and valve adjustment are reasonably close, it's not going to work right, and if pushed hard, will break.
While I disagree with some on the subject of motorcycle-specific oil (absolutely no evidence that car oil is a problem in motorcycles, not even in wet clutch applications), using bike-market-specific oil surely won't hurt, but it needs to stay inside the engine. Fix the breather thing, either by re-fitting the original, or corking off the cam chest vent and fitting either a hose or better, duckbill to the crankcase breather (crankcase breather is the one on the left, non-kick starter, side). Then the oil will stay inside the engine. If it doesn't and you're pumping oil from the crankcase, you have bigger problems, like no ring seal or a bad scavenging-side pump (which could also explain some of the top end problems; the scavenging side feeds oil to the top end). Check that you're getting oil out of the crankcase and up to the top end by loosening the fittings one at a time and setting oily hands.
If you can't do these things, find a lawnmower mechanic (I wish I was there--we could sort this out in an afternoon, then go for a ride), or get Snidel's manual, sold by our host. If your indy's mechanics don't understand Enfields, I wouldn't let them put air in somebody else's lawnower tires. There's not a lot of difference between a pre-Twinkie HD and an Enfield, except that the Enfield's simpler.