Yes, styrene. I think they were using it through the mid-1960s, but not exclusively. You will occasionally find an early 60s album pressed on vinyl.
Yes, if it wasn't distribution, maybe they either did smaller pressings and/or didn't leave things in the catalog for very long.
When you say "few items/artists were really scarce," do you mean because of the reissues on MCA, Jasmine and Fresh Sound?
Yes, I have that Wild One album, and it's a keeper!