At the moment I'm thinking about reverting the values of the Arabian and Sahara provinces back to vanilla, just because they're not exactly meant to be breadbaskets. Any objections?

nope

How can I edit the fertilty of regions in the startpos?
Can It be changed in-game? I would like to make the border regions of contest more fertile to attract more hordes. Right now they all seem to race to Italy or South France.

I finally figured out how to edit the actual regions in the startpos.esf file. This allows one to actually adjust a region rather than a province. Any interest? The hordes in my 1 trial were going to these regions like magnets. Have you tested your version to see if it affects horde movement?
I set the regions to triple fertililty that are on the borders and usually get razed. Did you do this in your mod ior just alter the fertility_level_tables.

Hi, I think, going from a bit of an error a while ago with the mod, that region fertility is accumultive, and it'll just show as provincial ferility. Though, I don't know if razing the region with higher fertility will cause provincial fertility to drop. Could be worth checking out.

As for the rest, yeah, I modded both the ferility level tables and start_pos regions.

So in Startpos, I gave each region a different fertility. In-game, I just see a large number that might be the sum of the fertilities. My question is if that fertility table or having a farm actually increases fertility.
For startpos, can I start the game with one startpos then switch to another one at another saveload?
You're welcome to borrow my idea of a mod where the border regions have high fertility that attracts hordes.

You can make it so buildings do increase fertility, but that currently isn't present in this mod. As for switching startpos', it doesn't work, since the campaign is generated based on the startpos, and that data lasts for as long as the campaign. Because changing fertility values is linked to start_pos regions it's impossible to change the values during a campaign.

And thinking on it, in the tables regional fertility is always for the capital region, so I don't think razing will actually change the values outside of the -1 for razing.

Changing fertility with a building would be a superior idea for the my purpose. Like putting out bait for the wolves. Can you think of the effect bundle that does that?