Can AddOns select a spell without casting?
 

Or anything in the action bar for that matter.

I get that the actual casting of a spell must be a hardware event, but is there a way to scroll through the spells/items in your action bar via key bindings or mouse wheel or whatever you'd want to use, and then bind a key to "use" ?

I didn't know if this fell under "targeting or untargeting any unit." And if it does, my bad.

I saw an add-on called Quartz Modular bar that says *Target/Focus Implementation of target and focus casting bars. I didn't know if that would be what I am looking for?

You want an addon that will queue up a spell for you so that you only ever need to push one button to do your rotation?

Well one key binding to cast or use whatever is selected in the action bar. And then have a previous and a next binding and finally, a universal key that uses whatever is selected. Instead of having to hit 1-5 be able to select one item via prev/next key bindings. But for all of that to work there would have to be a way to have something selected without it being used. So is that possible?

You might be able to do something with action bar paging to cycle through different spells for the same key.

addons are not allowed to change the macros/ bindings/ action bar page or any such things during combat... there are addons that can suggest what spell to cast next (like Spell Flash) they highlight the button that they believe you should press... if addons could select a spell you would be able to play WoW with only 1 button... the automation of the game mechanics is something Blizzard does not want addons to be able to do

Addons can cycle spells through a secure button in response to other secure buttons being hit, and that same button can be used to cast the current spell. This addon uploaded just this week does what the OP wants except it's for two spells per button and not more.

There's also this one for more than two, but it doesn't have a way to cycle through entries without right-clicking. But after seeing this thread and the surprising interest in the overall method, it's something to consider.