1) Impact on the sound would be negligible.
2) Would add maybe 2-4 pounds... depends on who sprays it.
3) Hard to say...
4) You'd have to prep it to bare wood.
The challenge though is that LineX, as a coating, is several times thicker than what is on there now. This does create fitment issues and I factor all those variables into my cabs, from the design process forward. My best guess is that after LineX, the grille likely may not fit.
LineX is a great coating if it is sprayed right but even then, there are still challenges and it's likely that there'd be issues, unless you are lucky to live near someone that has sprayed a lot of cabs. I wouldn't dare take even one of my cabs to a first timer.
Duratex is an option and is doable but results may vary and it may not look much better than it does now after a period of time... plus, you'd still have to strip it down to bare wood. The good thing with Duratex though is that it is easy to patch it up if it does get dinged.
I'm not exactly sure what RCF and Dynacord use but I suspect it is a 2 part system with a catalyst of some sort.