Many women use it vaginally, this is especially common in Europe.
IVF patients also use it vaginally, where the stakes are a lot higher, so that would also not be done if there were any concerns about absorption or effectiveness of this route.
When ingested orally, most of your micronised progesterone gets converted by the liver into other metabolites, so there is actually much LESS progesterone available to the uterus vs the same dose vaginally, which is absorbed into the circulation directly, into vessels in close proximity to the uterus.
Many products are used off licence indefinitely, and it is disingenuous for a clinician to imply that this is in any way controversial. I bet they would prescribe you an antidepressant in a heartbeat - that's not licensed for menopause either.
If there is any concern about whether your endometrium is adequately protected just get a private scan every year or so. This is good practice anyway and I personally think we should all have regular well woman scans like a mammogram or smear.
I would also think about vaginal atrophy if you are having pain on inserting the pessary and maybe try some vaginal estrogen.