No one answer- sorry.
Yes, any 98 stock will fit.
It will only fit the X7 and the A5. You can buy a model 98 version which will work with the 98 series, alpha black, project salvo and carver one.
A Tippmann carver one with e trigger.
You have to make one
They are the same gun, except for the carry handle, body look and number of rails. Neither is better, unless you want the carry handle (alpha black) or rails (carver one) There are literally no advantages to either over the other.
The new Azodin ATS and Tippmann Carver one are two you should look into for beginner markers.
No one knows what the purpose for the stock pen, theories are that the stock pen was used for extra guns,gun powder etc.
The best gun for any application is the one that you fits your hand properly and that you can shoot accurately.
Why would you want a plastic stock. I have this gun with a plastic stock and am considering replacing it with a wooden one. If I do, I will certainly be willing to sell the plastic one.
Is usually on the stock or the barrel if not you got a defective one
Your local gun dealer can order (and fit) one for you.