Two cents from a trainer perspective:
As you (aka. the ppl somewhat following discussions on the FAF discord) know, there is an ongoing discussion about the classic "how do I beat turtles?" or other things were trainers most often just give a theoretical advice bc we don't encounter turtles anymore so we can't show a decent gameplay.
With a challenge account it'd give us (= trainer team) a chance of showing exactly these type of things, similar to a thing such as "Bronze to GM series". We could play 1v1 ladder (yes, I'm basically asking for an exception), downgrade our gameplay so we play without manual reclaim and stuff and point out the important things. Atm it's the only way I see for really showing it cuz I've tried playing against low ranks with my main account, but they end up either being intimitated, trying cheese or simply not playing "standard", which we could avoid with an extra account.
For challenges itself, I don't really see a reason why you'd need an extra account if said account has a unique avatar basically giving away "oh hey, I'm a secret high ranked going to cheese!", could just use the own account for that. For me it'd require the account to be somewhat undercover so the other ppl play normal.