1. There is no such thing. As long as the engine runs well and does not consume a lot of oil, blow by is 100% pointless to worry about. The only time you should be truly concerned about blow by is during the inspection of a potential car purchase. Other than that, how much blow by your engine has is simply a curiosity. The only way to significantly reduce it (not possible to eliminate it) is to spend a few grand rebuilding the engine. If your blow by is only making the cap "dance", a rebuild would be a waste of money.

2. Not at all. Blowby is the passing of gas between the cylinder walls and pistons.

3. When it costs too much to fix/maintain.

4. Thats exactly what you should do.