the threads on this are for water / garden hose.
I see your point as a DEFINITION of "adaptation". This "adapter" adapts a 1" piece of PVC conduit to a 1" pipe thread to be installed in a junction box or terminal box. I believe what you want is a "reducer", and this can probably be accomplished easily with a 1" to 3/4" SLEEVE reducer and some 3/4" PVC conduit. Then terminate the 3/4" PVC conduit with a 3/4 in Schedule 40 PVC Male Adapter. Here is a link: https://www.homedepot.com/p/DURA-1-in-x-3-4-in-Sch-80-PVC-Slip-x-Slip-Reducer-Coupling-C829-131/203511880 HTHs...
Yes, that is what it is for.
Yes, pipe thread. You could actually thread this adapter into a galvanized, bronze of brass fixture.
Yes... its the same universal pipe thread and will fit any and all 1" galvanized, bronze or brass valve or fitting.
If it is a Dura brand fitting 436-010, the fitting measures approximately 2-1/4" from end to end. There are approximately 3/4" worth of threads.
For underground purposes, usually Sch 80 is preferred or necessary to meet code. Then the answer would be "indefinite", much like any and all other approved materials. I have personally used Sch. 40 for decades on un-permitted projects with no adverse issues BUT, all were glued using SLIP fittings, I never used a threaded joint and I would recommend you not use a threaded joint underground.