The proponents are the ones who trot out "the 40+ states do it without issue", however, when asked to list those states they become silent. More importantly, when asked to compare the state system the privates were absorbed in to in those states with the system as currently structured in Virginia you get nothing but crickets. The issue is one of fairness and a level playing field and without more details and ideas on potential implementation plans being made available it is hard to speak to the issue in an informed manner. Your last item mentioned about forcing them to all play 6A would make it a nonstarter for all but a select few private schools, however, it would sort out those that are truly interested in just competing versus those private schools looking to "have their cake and eat it too."
On the Tebow situation, if a parent is truly willing to establish a second household in order to attend a particular school, I believe it is their choice, not that I believe it is necessarily the best choice for family models. It really isn't much different than choosing to live in one area but, maintain a residence in another area due to better job opportunities or the family that takes up a second residence because it is beneficial to their child's development of a special talent or skill. I don't know how religously exempt students would be treated, however, at some point if you open the doors to one particular group the argument against not opening them to all groups is weakened, in my opinion. It all gets back to the same basic principle of a level and fair playing field for ALL participants regardless of which side you take on the issue, in my opinion.