If they had such abilities innately, they wouldn't need to become mediums or even pretend to be mediums, now, would they?

They could just become clairvoyants in their own right.

Impostor entities will not be able to produce concrete results, the same way a fake doctor cannot really treat the sick.

The proof is in the pudding.

I think you underestimate the power of a true deity, which extends beyond clairvoyance.

It is not easily matched by impostor entities, who in any case have no interest in aiding mortals, just in exploiting and preying upon them.

Aliens have no need for such rigmaroles.

I mean, look - there are TONS of popular comic books, video games and movies that feature aliens in some form or other.

The public is more than prepared to face the reality of an alien presence head-on, with no need to pussyfoot around the issue.

The bolded part is a big assumption.

For all you know, they merely want to enslave us or have us for dinner.