I'm not sure who you're replying to as I said FlatPak & Snap were solutions to a problem that didn't exist.
However let's consider your first two questions.
Firstly the whole idea of both those systems is that the basic package, Flatpak or Snap is to provide a set of prerequisite libraries for the applications packaged for those platforms. I'd expect any application that requires any additional libraries to have them included in the application package itself.
So we then have to ask can the base FlatPak and/or Snap packages be installed without manually downloading any pre-requisites. The packaging approach in both the RH and Debian based worlds has been for the system to automatically identify any addition packages in their repositories and include those, including pre-requisites of the pre-requisites. So let's see how I would do that on Devuan (which in practice means Debian for snything not systemd related) with the KDE desktop:
I can click to open my main KDE menu, select and click Synaptic, click Search, enter Flatpak and be presented with a list of Flatpak related options (that includes stuff for builders as well as installers, how many of your preferred non-twiddly options provide that), mark it for installation see a couple of required packages needed, click on Apply and have Flatpak and the prerequisites installed all without any use of terminals - should I so desire. I can do the same for Snap. No command line in sight.
One thing that might be slightly different from Windows, and, indeed, Ubuntu, is that on opening Synaptic I'm prompted for a root password. This is because Debian, like many Linux distros, follows Unix in being in principle a multi-user system and has appropriate security built in. Ubuntu differs in that it would request the user's own password as some measure of security. But even in Windows the system provides you with a warning dialog and asks you to click to approve, again as a sort of security measure. (I don't know what macOS does in this respect).
So in Debian/Devuan-land the answers to your first two questions is manifestly YES:
I doubt that things are essentially different in the RH/Suse world to what I've described above and I'm quite sure they're not different in the Ubuntu world. Again I would expect the answers there to be manifestly YES.
You can indeed go through the rigmarole of installing stuff from the command line. You may have read installation instructions on various how-to sites but for things which are in the big distro's voluminous repositories you don't have to.
The fact that you aren't aware of this makes it glaringly obvious that if you have any experience at all of the Linux world it must be a couple of decades out of date. It's always easy to spot those whose professed knowledge of such things is based on reading comments of others who are similarly out of touch.
You may well know what you're talking about for Windows and/or Mac. For Linux you don't.