If there is a way to do this in the current state of the game please let me know how to. I would like to be able to disable auto-attacking anything that comes within your attack range. Such an option would make the early game a lot less tedious; currently I've set up a macro to spam H to keep from attacking, but that's not really optimal.

It looks like there's a check box in the options that might do this but it doesn't have any apparent effect. Ideally, I would like to be able to stand there and not attack anything until I select a target to attack, after which it would repeatedly attack the selected target.

No way to do that currently. Closest is to tap H over and over as you said.

The option in the options screen stops the top target bar from coming up automatically when you attack something.

Yea i have trouble with this with 1 specific champion, Ashe. Late game when im trying to push a lane with my high dps, she will often randomly stop attacking when in range of several creeps.

U should be able to press Attack onto a point on the map inwhich ur champion will go to that point and attack anything on the way. This allows u to push a lane while watching what goes on around the map,

As mentioned it would be really nice to have a hold fire behavior toggle.

X is attack/move

Just keep yourself moving. It's a bad idea to sit in 1 spot anyways. Just makes enemy spells easier to target.
How would removing auto attack make early game LESS tedious? You would be sitting on the screen *waiting* for a creep to get low on health... Seems more boring than running patterns to avoid enemy spells.