1) Strip, lube, pad the spring until they're nothing more than 70rd locaps, then do the KA midcap mod on Arnies forum to them.
2) Any screw with the same threading, yes, assuming the mag's threading hasn't been stripped.
3) KA locaps reportedly feeds more or less the same as TM locaps. KA midcaps might require the modding you mentioned. For sniper application, locaps should be plenty.
4) Find the part number using your instruction manual, then contact the person listed under the 8th article on this page:
http://airsoft-news.eu/news.php?10 Or contact user JAFA on this board, same person, I think.