I think it's too convoluted, and many of us avoid WN for their boarding system (and I know many fly because of it). How long in your scenario before UA starts charging "EBCI" equivalents?
When seats are preassigned, most boarding anxiety is due to concern of overhead bin space. You know what fixes that? Not having to carry on a rollerbag to save $60 r/t. Free checked baggage is the part of Southwest the other carriers should be copying. And I know that isn't likely so my solution there is giving passengers a choice of one checked or overhead bag included in their ticket. When you check-in online and choose checked bag, you get Group 5 and take your time getting settled. If you check-in online and want overhead space, you get 3 or 4. It can be fairly easily policed because by Group 5 usually the overhead bins are full anyway, and once the GA calls Group 5 they would not expect many stragglers and thus shouldn't see roller bags (sort of like with E-).