#1, Not sure what you mean by “enclosed by custom tarps” I assume custom made tarp doors?
Can’t see why you would want to go with this instead of just putting up sliding doors. The hardware is not very expensive and the door are reasonably cheap and easy to build. They will easily out last any kind of “tarp” and never give any trouble in high wind.
#2 depends on how much “fill” is needed. Trucking in back fill, spreading and packing and or leveling an area is not cheap to do. It may be cheaper to build a “platform” built on post, kind of like a deck or pole barn with a floor engineered to hold the weight. You will need to do the “math”. Either way “pole” construction is the way to go. Not “stick framed” like a house.
If you level the ground you will still need to put something on top of the dirt, like used pallets or build a wood floor.
#3 more detail is needed. The overall size of a 75lb bale can differ depending on how it is made. The “math” is simple once you know the bale’s dimension.
If the bale is 3’high, by 3’ wide by 4’ long the volume is 36 sq feet. If your building is 20’ by 30’ and 10’ high the volume is 6000 square feet. 6000 divided by 36 equals 166.66666666 or just 167 bales. So that size structure would hold 167 bales of that dimension. But that is if all the bales are packed in tight. Realistically more like 150+.
Keep in mind that when hay is made the producer can set 2 things on the baler, length and weight. Height and width is “preset” on just about all balers. But the same “dimensional” bale can vary in weight. Hay should be bought by weight not by size. So you might want to “over size” the structure to make sure you can store what is needed by weight.
#4. On you own with that one. Never had to deal with “snakes on a plane” in my hay loft, storage.