Bottom line is that if packages in non-GNU ELPA are hosted on Github, like it or not, you are encouraging the use of Github. Yes, there are many Github features you can access from the command line and via other means, like commenting on issues via email, but these other mechanisms typically take more effort and are not as convenient (and have limitations - you cannot use markup when commenting on issues via email for example).
The non-GNU ELPA is supposed to be a repository for packages which are GPL compliant and it is a reasonable expectation that those who make their packages GPL compliant do so because they support the philosophical goals of the FSF. Therefore, I don't think it is too much to ask that they also have those packages hosted on a platform which also supports these same philosophical goals. As I understand it, non-GNU ELPA is not supposed to be a repository for all other packages where the author doe snot want to assign copyright to the FSF. It is supposed to be for all other GPL compliant packages where the author does not want to assign copyright to the FSF.
This will also have the added incentive of encouraging better hosting options. It might even encourage GitLab for example, to enhance their environment to meet Class B.
Many people have selected Github for hosting simply because it was the best known solution. With a little encouragement, they would probably be willing to move to at least GitLab, which offers many of the similar convenience features of Github. Being able to host your package in non-GNU ELPA might be that encouragement.
BTW I also think the questions regarding openess, arguments, not hurting feelings etc can be largely avoided by simply having clear well publicised policy which outlines the requirements for inclusion in non-GNU ELPA. The README is a good start, but it will likely take a few rounds of editing to get it right and make it clear (a challenging task, but not impossible) and a documented process for requesting a review for a rejected package or a package which has been included which someone thinks is not compliant.