It is hard to imagine a injury that disqualifies a mechwarrior from duty that sends them into a ASF. VTOL? maybe Conv Fighter? Yes.
Second question . . . bondsmen do not go through a Trial of Position when taken as a warrior. Phelan is the ONLY example and considering they dumped him into sibko training and in part his age . . . as well as being a first Clan book and, meh. Every single other example we have in fiction, the bondcord gets cut and the new Clan warrior climbs into the cockpit. Now most of those were older, but they were a mix of Inner Sphere and Clan bondsmen. Heck, do we ever get hints to his adoption ceremony again?
Off the top of my head, bondsmen turned immediate warriors . . .
Ghost Bear frees his Drac bondsman on Tukayyid, Shrapnel story
Phelan wins the 6th Jaguar Dragoons on Labthea, makes them all abathka & the 6th Wolf Guards before leaving the world- all ranks carry over
Ghost Bear frees a Drac bondsman, pilots a Atlas in a Trial against the Wolves . . . and IIRC, shoots his former bondholder's Kodiak in the back in a swamp?
Ice Hellions taking Falcon bondsmen during Operation Icestorm
Nova Cat Khan taking Trent as a bondsman before Great Refusal trial