I have a lot of confidence in the system, so I’m sure that something like what you are describing is probably pretty doable. Having said that, though, there is going to be a lot of “perfect is the enemy of good.” It will probably (in fact, almost assuredly) not work the way you are describing.
Not simply, but I’m sure there are ways to do this. Having said this, you might want to check “Conference Pro” which (IIRC) provides for in-conference key interfaces.
I wouldn’t call is a hybrid endpoint. It’s going to be a phone, or perhaps a phone app, that would be used via something that looks substantially like a POTS phone call.
I would expect that something like could be put together, but depending on your requirements, it could become a pretty substantial development effort.
The other part of this answer is that this gets into a lot of issues that have nothing to do with telephony services, like no recording on inbound calls and double firewall protection on the pay service.
I haven’t played with it in years, but we have (in the past) had the ability to use the “console” audio input and output. If you have access to the server, then this should be fairly simple.
No one is going to connect to anything but Asterisk, which is a Back to Back User Agent. Everything in the system is a single-leg phone call to the server, which may or may not then be bridged to another single-leg phone call (e.g., to a service, a phone, or another server). Because of that, you can set it up to do a LOT of really cool stuff, but they don’t always work the way you might think they do. As an Audio engineer (FCC First Class Radio Station Guy), I had to relearn how to handle these circuits. In my experience, it works more like a 2-meter repeater network, especially if you disable LMR-direct access from the radios.
So, there is a way for multiple people to connect to the audio stream. The simplest is to connect the stream to a Conference Call and have everyone’s “device” connect to the same Conference. Another way would be to provide an audio stream and use it as “Music On Hold”. In fact, those are only two of (I’m pretty sure) probably six or seven ways I can think of right off the top of my head.
So, yeah, we can probably get you pretty close. Some features are going to be “new” but most of them should be pretty ‘ready to go’ right now.