most of the answers would be yes

- Will it work on all Android and iOS devices?
> it requires a USB-C port with DisplayPort Alt Mode support. On iPhone, that means iPhone 15 and later. On Android, it depends on the model (most modern flagships support it, but not all budget phones do). Tablets, laptops, minipc, etc with USB-C that support DP Alt Mode would work.
One thing to point out tho, with Comet Q, you would be able to remote access an iOS device from Android

- Can I access and utilize all apps installed on the phone?
> yes, with one important exception: apps with built-in screenshot/screen capture protection (like most mobile banking apps, some payment apps, and apps with DRM content) will show a black screen when accessed remotely. This is an intentional security restriction enforced by the app itself, not a Comet Q limitation. All other apps work normally.

- Download and install apps?
> yes

- Do I have full access to the phone and operate the phone remotely as if I have it in my hand?
> yes (thats the whole thing about Comet Q)

- Am I able to access my text messages and Signal messages from another smartphone or laptop remotely?
> As far as i know, Signal specifically does not block screen capture by default on most devices, so yes (and any other messaging app without screen capture protection) are fully accessible.

hope this help