I see the 92A1 is a bit longer & heavier than the 92FS.

Will the 92A1 accept 15 round mags from a 92FS?

Yes

Yes. Mine came with 15 round mags from the factory.

Yes, definitely! They use the same exact 15 round mags as the 92FS and M9. I own a 92A1 (great pistol!) and have been able to find the mags on sale occassionaly for pretty cheap. I think I've paid as little as $16 for Beretta 15 round 92FS mags.

It will take them and love it.

its not longer at all. it may be slightly heavier due to just the rail

Magazines are pretty much identical. Beretta redesigned the follower/spring, and removed indentations to get the extra rounds in. Might have lengthened the magazine body slightly, like on the 11 round 96 magazines, but not 100% sure on that.

its not longer at all. it may be slightly heavier due to just the rail

 

Once I held a 92FS and a 92A1 side by side. The 92A1 felt heavier towards the front of the gun, I suppose the added weight from the rail. Not sure what the stats are on paper but that's what it felt like to me.