SymbOS allows to deactivate the desktop, so an app can work in fullscreen mode. In this case it's platform specific. An application has to detect if it supports the specific platform and then manage the video output by itself. This is done in the SymShell CLI, if you switch to fullscreen mode (which is currently not working on the EP) as well as in the screen saver. These slideshows would be possible in this mode
Handling drivers: LGB is right. Dynamic loadable drivers require more overhead and memory. For the CPC and PCW it wasn't necessary at all, for the MSX it was only necessary for the mass storage devices (things like mouse and RTC are standard here). Because of the additional memory and overhead I usually try to use built-in drivers, but if there are too many different devices, which should be supported, we need to handle it dynamic.
SymbOS should always run on original unmodified, unexpanded systems to prove, that it's possible
(in case of the EP at least with 128K and EXDOS card). This is even planned for the future. But as it's growing and growing, new versions will come with optional modules, which are loaded after booting the core part and handled like applications, but these modules still expand the functionality of the system. So you will still have additional new features in the future, but if you like to try it on an unexpanded system you just load the base version.
For the EP there will probably be two versions in the future: The actual "normal" one and the "power-user" one with higher resolutions, which won't run on 128K anymore.
The "Rom-version" of SymbOS CPC is only used for booting, that means instead of loading from disc it decompressed the data from rom. Running directly from rom would require a lot of changes (no self-modifying code anymore, task switching need to take care about the rom config, too). A rom version would have different code compared to the normal version, which will introduce a lot of more work. As it's not a problem to expand the ram instead of the rom, I don't see a reason for having a rom version.