No, the RPM application is not isolated because it is in Wayland.
They are more updated compared to your apt or snap app in Ubuntu and therefore have some hotfixes that you don’t have.
Yes, and in this case you may find some helpful information here.
Not recommended, by default you will be missing security that you only find when getting Snap in Ubuntu. Discussed in the forum before.
The security bottom line is that they are both somewhat secure and don’t offer substantial amount of security over the other.