The only arty piece that is trully OP in this game is Katyusha as it can consistently wipe lone squads on demand. It is tied to huge risk though because in order to achieve this effect you need to do barrage from minimal possible range.
Disagree with #2.
From my experience - static artillery pieces at best can be decrewed by counter barrage; self-propelled artillery may be destroyed in process if you don't stop its barrage if it got hit.
Recently I had one game where I decided to give another try to Priests. Had to stop my barrages after I fired 2nd shell as they got hit by Lefh for around half HP pool.
Counterbarrage is effective against both static and self-propelled arty.
i would nerf the katyushas penetration and i think it would be fine.