Hum..., I was checking your Thread 2 days later hoping you would have followed up a little bit, but nope...!
I guess a sunny WE keeps People away from the Forum, ah-ah...! Hum..., unless mentioning 3 Versions about your Env. proved to be a little bit too complicated, dunno...
But OK, here is a bit more Info, after re-reading your OP with a little bit more attention than my first quick Reading because of FCI not mentioned...
And yep, the Solution I had/have in mind should be OK in your Case, I would think...
The "Principle" would be to trigger an (Automatic) Conditional 'PROMPT' / 'WAIT' / 'PAUSE' when a Captcha comes up...
1- Depending a little bit "how" you run your Script, meaning if the "Human" Captcha Solver is watching the Script to pause it when needed, and depending a bit on the "Frequency" of the Captchas, or if the Script/IE Instance runs minimized or in the Background while the User might be doing "other things" meanwhile, the 'PROMPT' might be the most effective Solution because of its "Visual" Effect with blinking etc...
But a Conditional 'PROMPT' can be a bit tricky to achieve, as it behaves differently and needs to be implemented differently in all 3 or 4 Browsers supported by iMacros, because of some Bugs, broken Functionality/Syntax/Tricks in different Browsers/Versions... And again 3 days ago with the Release of v10.0.2b Beta for FF...!
=> Chances are that depending on which iMacros Version you are, when updating to some later Version, or updating IE, that it will get broken and you'll have to redo that part with some other Workaround...
2- Second Option would be a Conditional 'WAIT', of say 10 sec or 1 min, to allow the User to manually pause the Script in that time, if of course the User is watching what the Script is doing and is always ready to intervene...!
A Conditional 'SOUND' could maybe be coupled as well, to give the User some Audio Alert that they need to check the Script if they are doing "stg else" at that moment, and that they then have 10 sec or 1 min to take Action...!
3- And the third Option, my "Favorite" I think, even if it is the absolute only "Winner" that qualifies for a
"this is not really useful" in the Wiki, ah-ah...!
), would be a Conditional 'PAUSE'.
More Info in the following Thread for example, where some "exemplary" User managed to find it as well and to implement it very correctly, just from searching the Forum a little bit and reading a few of my Answers, ah-ah...!
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[Question] Set imacros to pause when ever a error occur
And that Conditional 'PAUSE' can also be combined with the same Conditional 'SOUND' like in Option_2, of course...
OK, see if you get the "Principle' and manage to implement it for your Needs...
>>>
But hum-hum-hum...
, having "seconds Thoughts" from trying to put your "give control to mouse" + "I suspect it's a "special" kind of pop up?" into a "concrete" Understanding of your Pb as your overall OP was a bit vague to be honest without any Script or URL posted and not (really) describing the Behaviour that happens (nor mentioning which "numerous Solutions" you considered/tried, like I already said), and if your Pb is about stopping/pausing the Macro or even once the Macro is paused, if the Mouse is not able to interact with some Popups..., but hum-hum..., if you meant the 2nd Case, then yep, I think I know what "it is" as well, and you'll be indeed hitting "stg else", ah-ah...!
But pfff...!, if that's the Case, try to be a bit more APAP and give more Details, it's always a bit difficult to try to help sbd / solve a Pb with so many "if-if-if" Assumptions..., I won't start to explain how to solve that "Part" if that's (again...?) a wrong Assumption...