Adjustable (carbine) stock on a 26" match rifle?

Yes, a carbine length buffer system will work with any upper. I have used my service rifle lower under my match upper - that combines a 29" barrel with a carbine length UBR stock.

An adjustable gas block isn't a hassle, it's an advantage. It's not needed to make it function, but it sure makes tuning your action a breeze.

The ACS is a nice stock, which offers a lot of cheek, but the UBR is much more solid. I have both on service rifles, but use the UBR lower as my primary. With either of them, you need a carbine buffer of your desired weight and carbine spring. The ACS mounts on standard extension tubes (both commercial and mil-spec available), whereas the UBR comes with its own tube.

Forward ejection is over gassing. I'm guessing you dropped a mil-spec weight carbine buffer into your rifle, which is under weight compared to your old rifle buffer.

Drop a heavier buffer into the rifle and you'll likely see ejection come back to normal, I like H2's in pretty much everything I build which isn't meant for racing. An adjustable gas block would also solve that issue for you - I run both H2 buffers AND AGB's in all of my personal rifles which aren't set up for Service Rifle (no AGB's).

Are you getting case head swipes as well? Unseated primers? Or just forward ejection? If you're getting CHS's and primer indications, then it's unlocking too early, so extra weight is the best solution. If it's just ejecting forward, with no early-unlock sign, then it's excessive bolt speed. An extra power spring would help, OR extra weight - and again, in either condition, an AGB would resolve your headache.