1. A perform-the-following-operation-for-one-turn command.
This is a great idea. I would use it a lot if I had time to play.
While the easy out is to add a bunch of 1-time actions a la Sentry Actions (Sentry for Land and Sentry for Sea), it would clutter the interface. Ideally it would require one new action with a button that you click and then click the actual action. I dunno if Civ4's system would allow this, but I don't see why not. The UI might not be super intuitive though.
2. IMO the group command is badly designed. . . . A possible implementation which is built on the previous command and leaves the current interface alone. <Once>-<Group>-<Heal>.
It sounds not so much like you want to use the existing grouping behavior but rather just assign a one-turn action (or normal action) to a group of units that they all perform individually. One thing I hate about healing is that if you have two units that require different numbers of turns to heal, you have to give the order individually or the one with fewer will wait in the group.
Can you clarify about the once part? Do you want this to be applied to one-turn actions you suggested above, or to mean more what I said where all the units are given the normal order individually, just more easily with one click?
3. If the current build is <unit*> and you add the same <unit>, then the <unit*> should be replaced with <unit>. As it currently stands, the queue is set to <unit*>-<unit>, which is rather pointless.
That should be pretty easy and makes sense normally. However, what if you have an Axeman, a Walls, and add an Axeman* to the queue. Should it replace the single Axeman? Or is this only if the immediately preceding item in the queue is the same? Probably the latter to allow for you to do whatever you want in the other rare cases.
4. A small bug, which is almost certainly known: if you click on a unit from the popup in order to add it to the queue, <unit*> gets placed in the queue instead of <unit>.
It's not a bug; it's a feature.
Because we cannot detect modifier keys in popups
the next best option was to make it a setting. Open the BUG Options screen and uncheck the
Build [Military] Units Forever options.
I still hope to be able to devote some time to modding in the near future, but at this time I can make no promises. Keeping up in the forum is about all I have time for these days.