in the Reign of Chaos manual, the High Elves you play in the Human Campaign were all volunteers from the Kirin Tor and Silverhand who ignored Anasterian's royal decree to leave the Alliance and return home; however in the Chronicles, it got retconned into Anasterian sending a few Priests after Terenas died
also, I daresay the Silver Covenant today's basis for rejoining the Alliance is their former allegiance to the Alliance of Lordaeron; and that they're the same High Elves who volunteered in Arthas' forces, as well as Dagren/Halakh/Magroth's refugee militia forces
majority of Kael's forces you play in Frozen Throne were from the Quel'thalas proper and only a few of them were part of the said volunteers like Rommath