1.- You have no add Guix to your PATH and reboot
2.- No this is not possible, you could install Emacs with Guix and then
launch this Emacs and it will have installed the necessary packages to use it
3.- I am not sure, it is possible. You can look for all the installed
packages using Emacs. Or looking for them here
4.-Remove the package with synaptic and install the Guix version.
Now for 1. How to add Guix to your path...
Did you follow step 8?
I'll save you the trouble it links here: