I thought you were giving up. 
but bitten by the bug.
Glad to hear it’s now working. The sim popouts are only the display portion of the unit. Touch is a major missing feature in the sim even though windows has support for it built in. To get round this a lot of us are using a free tool called POPM(Pop Out Panel Manager) whose main function is to popout, size and remember the locations of popouts. It also has extra features, one of which is pseudo touch support in the form of mouse emulation. It works well.
In Air Manager (AM) you will have to create a panel for the iPad and place on it the instruments you want. If you plan to use popouts on it any panel background image you use will need to have a transparent section cutout that corresponds to the size you have made your popout because the AM background and instruments sit above the sim popouts and would obscure the popout with a transparent section to allow it to show through.
Hope this helps. Trust me it gets easier once you’ve done it a couple of times.
Sigh - this seems exactly like something I’d want the younger me (the guy into ALL the tech and always on the bleeding edge) to help with. This me thinks - that sounds like WAY too much trouble.
The good news (I guess) is that I have a lot to keep me busy before then (today my eye thingee arrives, and Thursday I get a Warthog stand for my Alpha/Bravo controls - it also has one more stand that I could use for the iPad). I’m going to try and get all this working and together before I explore trying to get touch to work.
But - thanks for all the help. Given my age it’s very likely I’ll be dead before it’s all working properly .
If you want someone to talk you through the set up process of air manager and spacedesk, I would be happy to help with a discord. I could probably give you all you would want to know within about 20 minutes.
That’s very generous and I might take you up on it, but this holiday week I’m SO busy. Perhaps when things settle down next year you’ll be hearing from me .
NP. This is MSFS discord Microsoft Flight Simulator
Consider the offer open-ended.
If you want specific Air Manager from passionate groups of Air Manager users might I also suggest either of these more specific discord servers.