I don't really get what the problem is. Shooting off map can be avoided by checking the map as arty gunner. Do you mean spotting a high target next to an ABC line to circumvent the "no spot" dome on it?
1. The arty camera is directly connected to the target that the spotter marked. If that target is a high church tower, it will be marked high. Skilled spotters can use that knowledge by putting "high" targets for mortars and "low" ones for howitzers for maximum efficiency. I doubt that making it of fixed height would solve a lot of problems but it would definitely cause a lot of unnecessary ones like a lot of unusable spots and way less flexibility.
2. I like arty, as annoying as it can be on the receiving end. It's one of the last weapons that require a minimum of teamwork and communication in FH2 and I'd rather have it in, especially to deal with AT guns and the likes.
3. If you mean the ABC line abuse, a "dome" around the ABC line like PR has it could be an answer to that. To my knowledge, artillery and even airplane weaponry cannot pass through it if you're high enough but it won't stop the plane to fly through it.