i've never used that med so i'm not sure how long the full course of treatment is or the recommended WCs before redosing. i've used neo3 and neomycin/triple sulfa purchased from NFP and they're 10 day courses, 50% WC daily. the other antibiotics i've used have also been 10 days. no matter what, a full course is required, no stopping short because you see improvement.
sorry - rigid airline or airstone both work, i sometimes will run one of each and my horses tend to prefer the airstone, kind of like little bubble massages.
i keep at least a 5 gallon bucket of new water brewing at all times when they're in QT. it helps to have prepared clean water ready if needed. it can be stressful treating this guys and the last thing you need is to be frantically mixing and trying to match parameters. ammonia can be a problem because there's no biofilter so watch out for that, another reason to have matched water ready.
it's good that she ate something, that is one of the keys to treatment.
your plant looks like one i have in my QT tank now

. i also have a piece of clear acrylic tubing (about 1" diameter, feeding tube kind) attached to the back with a spare heater holder. anything that's easily cleaned and they can hitch to comfortably is good.
hard to say why it happened. when did the first symptoms appear? was it immediately after you added the second female (she's pretty)? or did she have symptoms prior to the new addition (suppressed appetite, tail beginning to change color, etc.). as panmanmatt said the other horse could be an asymptomatic carrier. could just be she's always had it and for whatever reason it popped up (stress, temp, ammonia spike, injury to the tail).