(1.) Which apple varieties are the obvious choices to pollinate a Gravenstein apple? (2.) Is there a single apple variety that will do the trick (a self-fertile variety)? (3.) Is there any cross-over between a Gravenstein and any pear variety? (4.) In other words, what are the pollinizing options for this early blooming and early fruiting apple?

I have 3 "and a half" Gravenstein Apple trees. The main pollinator used prior to my arrival was "Transparent," I believe a "Lodi" is about the same thing. After removing most other pollinators in the orchard, and learning that a Gravenstein requires pollen from two other apple varieties (I've never heard of a Pear intervening), I researched some early bloomers.
With a "wish-list" of 5, I found every one at our Spring Scion Exchange! Of those I grafted, basically using the best sounding apple and the best looking scion wood, "Summer Red" just took off! It consistently sets full apple "clusters" like grapes! With their contrast of red, to my old green Gravensteins, everybody's grabbing for them! - except me, Gravensteins are still the best!!
With those two pollinators I now spend hours thinning golf-ball sized green Grav's to keep the trees from falling into a "Biannual baring" pattern - meaning; way too much fruit one year, nearly none the next - do thin.

Traditionally, Yellow Transperent has been used to pollinate Gravenstein. Warren Manheart recomends Erwin Baur because it also is a very early bloomer and is of high quality.
Most crabs bloom very early too

> ...and learning that a Gravenstein requires pollen
> from two other apple varieties...

I don't believe this is true, though I certainly could be wrong. I think the confusion comes from the fact that if you have a Gravenstein (that has sterile pollen) you should have two other apple trees. So that the other two trees can pollinate the Gravenstein as well as pollinating each other. Without the third tree, the Gravenstein would be OK but the other would not get viable pollen from anywhere.