You sound like a gal that is trying to do the best you can for your horse and you are very attentive about.
When using your sand clearing products don't forget to divide the recommended dosage by 4 since you are administering to a mini. And again, I'd de-worm him now before I'd blink an eye.
Don't give up on the beet pulp. Mine hated it for a long time. I think it's an aquired taste and it smells too. P U !!! Just make a sloppy mash of it in a small quantitiy about a cup and ease him into it and eventually, he'll go for it. You can add a bit of applesauce to it. Presently with this heat wave, I'm mixing mine with gatorade and they are loving it. I use it as their noon time feeding in the barn to keep them occupied in there during the heat of the day. Don't forget to rinse out that beet pulp bucket as soon as you are done because it lingers and sours in there something pitiful.
I get the idea that all you have availble is that little pen to keep your little guy in, so if that's all you can offer him right now, then I would go with a nice short daily walk with him about 10 minutes to keep him from going stir-crazy in there and getting depressed. Too muich confinement in a tiny area will only cause mental problems so as long as you are not excersizing him, go for the walks and enjoy the scenery with him or just sit under a nice tree with him while he grazes.
I didn't know what Chaffhaye was so I looked it up. Sounds good but probably too good for what he needs at the moment. I would again encourage you to do some serious traveling for some decent grass hay as I understand there is a shortage this year in many areas. I"ve even had to go out of state in the past for good hay. No fun.
Here is the link for anyone else that isn't familliar with it:
http://www.chaffhaye.com/
Best wishes to you. I think you are going to make a very good horse mom. Seems you are on the right track.