First and foremost! Welcome back to horses! They'll keep you young at heart, but also remind you the ground gets harder and further down every year.
Let her have all that salt and mineral block she wants. She won't get too much off a block - in fact, she may not get enough. There's some debate about how effective mineral in block form is for horses. However. We keep them in the pastures for our horses, but also our cattle share the same pastures and have mineral feeders. The horses AND the cows get both block form and loose mineral free choice, and it's not too much. We keep a yellow block (sulfur/mineral/salt), a brown block (Mineral and salt) and a white salt block available in all pastures. I see our horses and our cattle picking and choosing what flavor they want on a daily basis. I keep a cobalt salt block for the Old Man in his pasture... it doesn't seem to be a popular choice. Supposedly the cobalt helps with intestinal health as a horse gets older. I wouldn't bank on that. The yellow blocks - sulfur is supposed to help with flies. I also wouldn't bank on that, but since putting out the sulfur blocks this summer, I've noticed a drop in black cattle flies. Correlation does not equal causation though, so take that with a grain of salt. Regardless, let her have all she wants on a salt block. She'll eventually slow down on how much she's eating off it. If she's treating it like a box of chocolates right now, she probably needs it. You're going to want to get loose salt/mineral for her and either top her feed with it or invest in a mineral feeder.
I can't give any advice on the type of hay as ours stay on a native bermuda and bahia grass pasture in the summer, then bermuda/bahia hay in the winter. I don't buy square bales except for camping since, again, the horses and cows all share pastures and we feed with round bales. Square bales here are almost always bermud/bahia mix, so that's what we roll down the road with. I do know a first cutting of anything is going to be rich in terms of sugars. I've found if I feed one of our horses a first cutting hay in winter, they get the poops because it's a shock to their digestive system, they aren't used to the richness of it that late in the year. Someone here with more knowledge than me can advise you on that - we don't buy hay until the last few cuttings in late summer since we don't put out hay until the grass is gone.
I'd offer your girl free choice, all the time, on hay, once you settle on the right hay for her. Others here prefer to give x number of flakes of hay in the morning and evening. We just let them have all they want, all day long but they aren't stalled either. Hay takes energy to burn and that energy warms a horse up and helps get them through the winter cold, so that's why ours get to stand with their head in a hay ring all they want in the winter. :P
A plastic pail of hay will not be nearly enough once the grass starts to either burn up from the last of the summer heat, or runs out as fall approaches, or dies at the first frost. You're going to need a manger or a hay ring type set up. I'd look on FB marketplace for a used one, or a hay feeder for horses (Has a manger on top, a trough on bottom). You might can find them made new locally or even used on there. Those selling used will often negotiate on price. You might also hit up any farm/ranch/estate auctions in your area if those are a thing.
Feed, I'd keep using the senior feed given her age. As to how much? I can't tell you that, each horse is different, but stay away from the sweet feed (Which it sounds like you're doing). Be sure you don't overfeed with grain based products. I will say this: Our senior horse that's become a hard keeper gets 5lbs in the morning, 5lbs in the evening, and water soaked alfalfa cubes twice a day as well. This is not a maintenance plan as he's lost a lot of weight this summer and we're fattening him back up. This is what the vet okayed for him. What your horse needs in terms of volume may be entirely different than our Old Man needs.
When winter sets in, The Old Man will be getting his senior feed topped with rice bran - it's a good, cheap source of fats and he really enjoys his rice bran toppers. Once we hit the late part of winter, I'll be topping or mixing all the horse's feed with it. Local feed stores usually sell it for less than franchise farm and ranch stores. If her teeth are good, I'd also consider alfalfa pellets - not the cubes, the pellets. Local feed stores will often have them in a very fine size - No 2 pencil or smaller sized, not much bigger than extruded feed and, at least where I live, it's half the price per bag of feed, so I supplement winter feeding with those for the horses with good teeth (Everyone but the Old Man).
You might consider getting her a protein tub formulated for horses. I had the 33.3 gallon one from Tractor Supply
recommended to me by a friend who takes in rescue horses and gives them a new lease on life and an education. I now keep one out for our senior horse at all times. He shares his with an 18 month old and 3 year old. We've sold about half our cattle and they will be leaving Saturday, which leaves me a pasture open for nothing but my mares and fillies - once the cows are gone this weekend, I'll be keeping two or three of those tubs out for the girls through the autumn and winter as well. Do not buy one formulated for cattle. They're cheaper, but they're too 'hot' and have ingredients that are unhealthy for horses and as our local ranchers say: It will burn a horse up.
Water, IIUC, Arabians drink less than say, a quarter horse, due to their desert origins. Still, be sure you keep water available at all times and that it's fresh. We have ponds, but in summer I also keep a 200 gallon water trough clean and filled with fresh water for our senior horse so he doesn't have to walk his weight off to get to the pond at the other end of the pasture. Trough is in the pen where he gets fed and just a few yards from his loafing shed.
I'm sure others here will chime in and they may have far more comprehensive answers that I did. So! Good luck! And once again, welcome back to horses!