From the pics, you have done a fine job of bringing her into MUCH better health/condition...congratulations!
Sounds like you are on track with proper discipline; I agree w/ most of the advice given about quick, sharp correction, and not allowing behavior that may seem 'cute' or 'ok', but may lead to MIS-behavior(agree not to encourage or even allow, licking).
You mentioned her 'going through' a bale of alfalfa in 10 days? If your bales are anywhere NEAR the size that most alfalfa is baled around here, that sounds like way too much for a 30" mare! I am an advocate of alfalfa, have fed it for MANY years, but found it should almost never be the 'only' hay, nor should it be free-fed. The slow feed hay bags/nets are a GREAT idea, but are made to work best w/ grass hays; just due to the 'nature' of alfalfa, they are not going to limit access as well as you need. I would recommend feeding BOTH a decent grass hay AND some alfalfa. Put the grass hay into the slow feed bag; smaller portions of alfalfa could be fed in an ordinary feeder. I like about a 30% alfalfa, 70% grass proportion, but that can be adjusted either way; in the case of a youngster such as your mare, you could use a little higher proportion of alfalfa. Horses really NEED that 'continuous 'grazing' experience; both physically and psychologically; though I don't much care for the idea of grazing muzzles, if she needs it for the reasons you've cited, keep using it as you are...but then, keep a slow feed bag of grass hay available at all, or nearly all, times. With continuing good nutrition, maturation, proper exercise, and an ongoing correct deworming program, any 'pot belly' should really take care of itself over time, IMO.
It is true that young stock needs a higher protein level than older horses; you might consider a quality Jr. concentrate. I am currently using and REALLY like, a FORAGE-based, fortified w/ vit/min/required nutrients,steam-processed, light 'kibble' feed called 'Total Equine'; I believe it is available around the country. I really prefer a similar feed called 'Thrive', but it is hard to obtain outside of TX and a few places in KS and CO. If it ever goes to wider distribution, I will go back to it!! You can google either to read more about them. I also regularly feed a stabilized ground flax, and see results in hair coat and hooves.
Your filly seems to be the 'I-need-to-always-be-doing-something' type; lots of turnout time, exercise, all the 'toys' you can think of that she can safely play with(I have had good results just tying a length of stout hard-lay cotton rope to the fence/overhead, w/ knotted ends, as something to 'mess with')--all to give her something to DO with all that energy...should be useful.
Not sure why it would be thought that horses 'only' rear for two reasons? Yes, fear AND resistance are likely two reasons, but certainly not the only ones...and let's face it, we can't REALLY 'read the minds' of horses, so who can know what ALL the reasons could be?! I do agree, allowing one to rear on the longe line, or ANYTIME it isn't a carefully-taught 'trick' under suitable circumstances, is a bad idea!
Best of luck with your horses(GORGEOUS markings on the paint horse, BTW!)
Margo