I have no idea regarding the grass or dandelions. It's not something that I've ever used as a food source for my rabbits. We keep ours strictly in-house and there was always such a nice variety of accessible foods that it never occurred to me to try it. :
I imagine you've already done your research regarding an exotic vet (many vets do not treat rabbits, and of those who do, not all are created equal). What I would do, regarding your dietary concerns, would be to contact their office and ask them. There is a tremendous amount of misinformation on the internet and that can be very frustrating when you want to give your pets the best life possible. That being said, I have never had a vet who recommended a pellet diet at all. That's actually something that I didn't learn until I brought in my first rabbit in with GI stasis. That first vet said that we should say our good-byes, so I went to the equivalent of an emergency room for animals. The exotic team saved his life that night, and I received quite a dietary education.
Both of our GI stasis rabbits eventually went on to live beyond the 10 year marker. There could be other causes (excessive grooming causing hair to block the intestines, for example), but a heavy pellet based diet was the more likely cause for both guys to fall ill. Pellet fed rabbits are also more likely to be overweight. I don't know the science behind it, especially when so many brands claim to be made totally of ground up hay bits. :
I recommend that you do fix your animals, especially if you ever plan to have more than just the one. There's a reason that they say that something "multiplies like rabbits", and it's not uncommon to think that you brought home two fluffly little girls/boys from the breeder, only to find a cage full of pink babies a few months later. : Fixed animals are also much easier to introduce to each other. Ask Charlie... Really... We worked on making hers get along for MONTHS.
A fixed boy and a fixed girl make the best pairing. Two males will fight like Hell, as will two females. I'm not saying that same gender pairing isn't possible, but it's not a lot of fun troubleshooting. I've had rabbits that never learned to get along, and I've had pairs who were inseparable. It's all in the personality. Pairing rabbits, especially when one has been an "only child" for an extended period of time can be stressful. Once you have them bonded, it's so worth seeing their friendship and love unfold.
You say that your house gets in the 80's. Are you planning on keeping him outside in a hutch, or inside the house with you? Does your home have air conditioning?
He'll chew through cardboard like a candy. You'll have to find something sturdier to keep him from sneaking through the holes. They're REALLY small when you get them. If you're not sure, find something with less space between the bars. I carried my baby (he has many names, but mostly we call him June) home from the breeder in a HAMSTER cage. I used to leave the door of that cage open on the floor of my living room and when he needed a rest, he'd run in to sleep in the wheel.
As far as how often do I feed him fruits or treats... We don't usually plan it. He's at my feet constantly, when I'm in the kitchen. If I drop something, he's there before it hits the floor. He eats a lot more than a spoonful twice a week though. His particular favorites are brussel sprouts, small amounts of kale, ROMAINE (not iceberg--iceberg lettuce can make them very sick) lettuce, apples, bananas.
Even though he shouldn't have them, he also finds the occasional piece of cereal/cookie/crumbs/bread on the floor. You'll never vacuum or sweep again!