How would you pick Tespia participants?

I'm sorry but if you're below 120 how would you even know what important glitches to look for in terms of the larger amount of gameplay. The level requirement is fair because it comes with experience. Quests are pretty irrelevent and can be fixed easily, glitches unreported could take up to half a year to even be looked at..

Both high level and low level content needs to be tested, and by appropriately levelled characters.
How would you test Episode 1 (GMS-exclusive content, btw) on a level 150 quest specialist? Even if the level caps for the quests and portals were removed (which would open the door to different bugs), if you one-hit Andras how would you be able to tell if there's something wrong with its attacks (e.g, 10 minute stun instead of 10 second)?

This.
If you have never experienced 4th job how the heck would you scout that content for bugs and general oddities? It's not like you have experienced it until you have the skills mastered in terms of usage and know how the high level areas and bosses work.

I believe in the copy-characters-over model of Tespia, not the level-up-NPC one. You know your existing character, whether it is level 50 or 150. Changed skills and mobs are going to throw you off to begin with, but you are still best qualified to test content meant for that job and level.
Maple has a lot of low-level content to be tested, including the entire character creation process, so I would not disqualify someone who only has low level chars. They will only get to test low-level content, that's all.

What you do is hold registrations (like KMST) for those who WANT to participate in Tespia. Then you have those who register, on their applications, explain in full detail why they want to participate and how efficiently they would search for and report bugs. Then you proceed to pick the top applicants from the list to be your testers.

It's a more work-heavy process, but it would definitely turn out better than choosing people randomly, who probably don't even want anything to do with a test server.

Questers are more likely to be sufficiently inquisitive to want to check out the new content. That's why I'd prefer them to grinders or socializers.
Using applications can yield much better results, but, reread the OP. I am assuming Nexon is too lazy / short-handed to go through that process. I'm trying to suggest something that wouldn't take much more effort than a random drawing, and still give significantly better results.