1) I feed twice a day, but my boy is paddocked during the day so has free access to pasture. When he is staying at my coaches place he's in a yard and I will allow him ad lib meadow hay during the day. But he is still hard fed twice a day.
A stalled horse is best to be fed 3 times a day though, it IS healthier for them to be fed three times, to split the meals into smaller portions and keep their gut moving. A stabled horse should have constant access to hay anyway.
2) When I was at school I didn't have a problem with keeping my horse riding going and grades up. My horses are kept at home, I feed them twice a day, rug, etc. etc. The only time I decreased my riding was in the last term and a bit of year 12, when exams were coming up. Otherwise no problem at all. If it's not horses it'll be boys/partying! (or if you're like me, all 3 :P)
I'm at university now, and more serious than ever about my horses. I work, have a serious boyfriend, study full time etc. and I am managing. I have exams coming up in 2 weeks so have dropped off my riding a little to give me more time (Usually ride 5-6 times a week!) but horses certainly haven't detracted from my grades.
3) It can do. Really depends on how much you are doing with your horse. I'm usually riding/lunging/mucking out/grooming every day sometimes twice a day and that helps me stay reasonably in shape, I'm fitter than a coach potato at least. But horse riding alone won't keep you truly fit. I like to run and do some weight training occasionally to keep in shape.
4) I don't regret it by any means, especially not with Hugo. Every horse I've owned has taught me something, particularly as I have a habit of 'pity buying' and landing with nice, but very green horses. Only one horse I've come close to regretting. I was ripped off buying her from my then 'boss', who told me she'd dog her otherwise. I bought her out of pity, trained her to elementary dressage, fixed her medical issues etc. and got $300 less than what I paid for her!
But overall, no I don't regret it. They give me so much happiness, provide me with a goal, a reason to get out of bed (whether I like it or not :P).
But yes, horses are a BIG change to not having a horse. They are A LOT of work, do not underestimate how time consuming and expensive they are.
5) I ride 5 to 6 days a week. That is my current horse, I will often ride a couple of times a day depending on whether I've got another horse in work or not. It really depends on what you are planning on doing with the horse. If you're a pleasure rider, a couple of times a week is plenty. For myself, I am a dressage rider and want my horses fit and strong, and riding 5-6 times a week helps this as well as progressing quickly with their education.
6) I generally like to have a round bale of average quality meadow out in the paddock for my horses to pick at. But my horse in full work gets 2 biscuits of quality meadow hay every night with a hard feed, a hard feed in the morning and is paddocked all day with good pasture so I'm not having to feed so much hay. Although he is an extremely good doer so the amount of hay you feed does depend on the horse. I've had a few that I was having to feed out almost a full bale every day to keep weight on.
7) Grain isn't essential. I hard feed my horses (a combination of chaff, processed feeds and barley/oats depending on the horse) when they are in work, those that are not in heavy work and are nicely weighted can be quite happily living off hay. Many horses in light pleasure work are perfectly happy on hay and pasture.
However when a horse is being regularly worked, to the point of sweating and blowing, I like to hard feed them to replace many minerals they lose, and to provide energy and assist in muscle building.