Borrow-A-Hero Tips & Mechanics Explained

I'm pretty sure you are only prevented from borrowing that particular character from an ally for several hours once you use it as an ally. You can borrow from that ally again if it's a different character, e.g. if you do a dark side mission and use one ally and then use a different ally from them in a light side mission or if they do a mission with Qui-Gon as lead, you borrow Qui-Gon, and then they do another mission with Luminara lead, and you can borrow Luminara before the timer expires on Qui-Gon. I've done that with my own main account and alt account, and I've managed to borrow several characters from the same account within a half hour. I didn't know about the reset from doing gear challenges, but that would never affect me, because I do those in the morning, and then I do my farming missions for shards and gears, which I usually SIM on my main account, and I never get to trying to work on new missions that I haven't yet beaten until later in the day, when I've got some energy left but am further down my list of characters I want to farm and decide to get further along in opening up more missions.