realskiers

I wouldn't worry about the geometry changes in the least. 20mm is a small change, and with the increased travel you will probably be running a little more sag too, so it might be a change of less then 20mm. You may also be able to get a headset with lower stack height which would minimize it further.

WRT the wheels, you probably don't need an adapter, you may already have an adapter - from 15mm to 9mm. To find out if you do, just take out the QR halfway, and see if you can push it cockeyed, then whack it with a mallet and see if the 9mm endcap pops out leaving you with a 15mm thru-axle repeat on the other side and you are good to go.

I'm kind of a Fox guy, but any of the forks you are talking about could be good. The Magura is pretty intriguing. In terms of having customer support though, I'd say Fox and RS are your best bets, the others all seem pretty shaky.