1. Dirt will actually help your plants grow. It contains minerals and nutrients that plants don't find in sand. Sand is just sand, it's all grains and has no nutrients in it. However if you do use dirt, you'll want to use a sand cap over top. Dirt is really good at floating and clouding up your tank so if you have about an inch of sand over top of it, it will stay down and keep your tank cleaner. Then you plant the plants in the sand and they reach down to the dirt to keep them fertilized. You can actually use Miracle Grow potting soil as a base with regular sand on top.
If you just use sand you'll have to use root tabs and liquid fert (it's really not that hard to do) and you may not even need root tabs right away or anything. I successfully grew plants without root tabs and with API LeafZone for 6 months and they're still doing great! I only added root tabs for my Sword plants since they are such heavy root feeders that they really needed it. They were getting floppy but this is only a sand tank.
Disadvantages for dirt/sand is that at first it might be cloudy for a while, you do have to poke the dirt/sand to aerate it. Snails won't do all of the aeration for you so you'll just have to "burp" it every now and then.
2. same way you clean a regular tank. You can use a small siphon (there's 4 inch siphon on ThatPetPlace.com if you want one) and just instead of digging like you would in gravel, you just skim the surface of the sand to bring up debris. Sometimes I swirl it around gently to kick up debris and poop when I see it.
3. You can, but you'll still need to aerate it yourself as I mentioned before.
4. I don't know, I don't know what you know already haha
Leopards: 1:4 Hawkeye, Luna, Astrael, Dorian, Cullen