I would suggest going by the statute's default of one vote per parcel, and I also like the definition of "parcel" as being "lots with an owner in common." Our road association had one developer who owned several undeveloped lots. We considered billing him per lot, but felt that if we did so, we would also have to grant him one vote per lot, giving him more leverage with the association than would be fair to the other owners. We considered it again when a builder bought three of the developer's lots and put a house on each. But again, billing him for each house would have given him three votes. Besides, he was one of our non-payers anyway. So in the end, we just counted him as one owner with one vote, and billed him for one share. As he sold off the houses, the new owners added to our income by increasing the number of separate owners.
But in his case, the houses were vacant while the builder still owned them, so we were not contending with increased daily use of the road. (We did, however, have issues with wear and tear on the road during home construction - but that's another question.) If one owner has two houses and they both have people living in them and using the road daily, there should be some provision written into your assessment "formula" to allow you to charge for each inhabited dwelling.
As for the "part voluntary, part statutory" road association, I don't know why you couldn't do what we did with ours in a similar situation. We had a non-profit association, but people found out we had no practical way to enforce payment, so each year we lost a few more. With fewer payments, we were able to do less, resulting in fewer people being willing to pay. In the end, with only twelve members paid, we folded the non-profit and started over again as a Statutory road association. It's taken some time, but we now have only two unpaid members out of 53. It doesn't make sense to have two road associations on the same piece of road. If they were both functioning, that would be redundant. If one of them isn't really functioning anyway, it should be terminated, or any remaining functions assumed by the new association.