Improvisations are entirely subjective -- as one has to depend upon the AHJ seeing things your way.
Your description is unclear. Do you mean the oil ring is on the outside of the NEMA3R gutter with the lock nut inside?
I've seen silicone used instead of an oil ring. I never new if that was inspected work or not.
In cold weather PVC contracts and becomes quite brittle. Throw in ice heave/ stress, and it's not a material that I'd favor in snow country.
More generally, it's a poor time-value descision to try and super-value-engineer assemblies -- particularly any assembly that is fully exposed to the elements. You stand at serious risk of getting shot down by the AHJ -- or by your one-year warranty obligation.
Your best time-value return comes from being very efficient and above all, not stumbling over materials/ materials shortages. The amount of time/ labor wasted because of materials shortages/ materials hard-to-find is leagues beyond any clever assembly scheme.
As for style points: anything that is highly visible to owners/ customers/ GCs should be assembled to look professional and handsome. Such work becomes a sales aid forever afterward.
This is really more of a listing question than a code question. You could ask the installer to show you where that connector was listed as being rain tight. If it was one of the male adapters like you get on a liquid tight assembly with the O ring, it may be. I believe the installation instructions show the O ring goes on the outside between the shoulder of the adapter and the can.