my melee toon will run up to the mob, but if the mob moves, my melee toon will just stand there out of range and not attack, unless I spam the Interact-with-Target shortcut again. If anyone has a suggestion for getting past this, I would appreciate it. (It would be lovely if I could get the melee toons to behave like the hunter's pet and actually stick with the mob until it's dead.)
There is no getting around it, IWT is not like autorun. You must reissue the command every time you need to move to a target. You can add it as one of your steps, but that's a whole other can of worms.

(I know I could get around it by making the melee toon my main and driving that toon, keeping the ranged toons as slaves, but the only problem with that is that it brings my ranged toons too close in on combat. Interact-with-Target pretty much has everyone except my hunter trying to melee auto-attack)
You need to break follow before running the melee up to the mob and your ranged will stay ranged.

if I press 1, all the toons will trigger a cast-sequence macro for basic spells/attacks (preferably laying DoTs first) but the melee toons will get in range of the mob while the ranged toons will hang back.
If you press 1, 1 thing should happen, not a sequence of things. You can use steps in which the first time you press 1, the first spell goes off, the second time another spell, the third press does IWT, etc., but it will require your physical input for each action.

all the toons will use AOE spells (not the kind that require setting a target on the ground, of course, unless there is a way to automate setting that target around the mob(s) being attacked or around the melee toons.)
You can not do anything that requires automation. ISBoxer has mouse passing that you can manage manually. Many people use it. For spells that don't require a ground placement, use them just like regular spells.

the toons with heals will cast a general group-heal to top everyone off (since most of the time in PvE leveling, I won't have anyone getting low enough in health to need intense healing.)
You don't even need a macro for this, just put the spell in slot 3 on your healer toon. It should "self cast" and heal the group without even needing a party member targeted.

Instead of using Alt-G for "Interact with Target" with the quest NPC, I'd love to be able to put that function on the Hotkey bar.
IWT is a function that can't be called with a macro. IWT is also not a command that you can send to any toon but yourself, putting it on a bar on your main will not make your slaves IWT when you press it. If you don't like the IWT keybind, change it to another key combination. While I don't use ISBoxer, I imagine the "Interact with Target" key is also tied to assisting and possibly toggles click to move, so you will need to find that keymap to make the changes.

I'd also love a macro that would have all the toons grab their quest loot from a corpse instead of me having to switch toons and have each one loot individually.
This is what IWT is for. It can be used for -

interacting with an NPC to get a quest/take a taxi/vend/repair/etc
interact with a mob to attack
interact with a corpse to loot
interact with an empty corpse to perform tradeskills on it if applicable {skinning/herb/mine/eng}

Just like with NPCs, select the mob on your main, have your slaves assist and then interact. You may need to spam it on quest pickups because of the "That object is busy" error, but it works well.

And another for mousing over a corpse/herb/mineral node and skinning/herbing/mining.
There are no macros for doing this, you need to use mouse passing and very few people use it for gathering because it requires specific placement and a lot of setup. I do use it for gathering and even fishing, but mine is configured with HotKeyNet. Someone else may have adapted it for ISBoxer.

Much of this stuff is covered in the Newbie's Guide {One | Two} or links within the Newbie's guides. MiRai just posted an entire video series on setting up ISBoxer and has tons of other videos covering just about everything there is to cover.