Running Firefox in sys-net is removing all Qubes-specific protections, making Qubes almost as secure as Linux. Nobody will stop you from doing that (you have all the flexibility you want), but I suggest to explain to beginners that it’s a bad idea.
Unfortunately not everyone has enough time to try every wrong configuration and perform many reinstalls. See the link above. I believe that decreasing the number of required reinstalls would greatly increase the Qubes OS user base.
Yes, definitely. The explanation of design decision behind the default qubes is exactly how the community can help.
Not sure what you mean.