Very interesting read, Although light accounts would vastly improve the useability of such system, it is definitely doable right now.
You mentioned an unlockeable second account with little rc if you reach enough sp, isn't this exactly what claiming an account with rc does ?
And if people were to run a steem patreon, assuming that people will basically do one transfer per month, it would need almost 0 rc, so a claimed account with 0 sp would totally work for that. A dapp can already do this and profide free claimed steem accounts to anyone who signs up.
And assuming the dapp ran out of accounts, if a user is ready to put even one sbd per month for a content creator, the dapp can definitely take a bit from it's first contribution to pay for a steem account.
Basically light accounts would be needed to scale, but it takes time for apps to scale so I'd say that claiming accounts totally works for now for such a dapp.
And in the end do you really need light wallets ? Most of the users won't care about steem itself, or simply will be in a country where it's hard to buy some. Why not just give content creators who sign up wallets (claimed accounts, which is now a manageable number since there are way less content creators compared to contributors) and then setup automatic payments with paypal/whatever to buy steem/sbd and send it to their wallets ?
The fiat part will be centralized, but anyone is free to contribute to those content creators via their wallets later on. It seems like an easier route, except for the legal implications of taking in fiat and buying crypto with it.