I don't think your specification involves any purchases using Steem -- that is where soft-consensus falls short, when transfers with Steem must be incorporated into the DApp (I do have a solution, though, but it is quite complex and involves pegging a token running using soft-consensus to STEEM). Anything other than incorporating STEEM transfers, though, can be done using soft-consensus.
But is there any part of your specification that actually requires native consensus? I haven't read through it all, so sorry if I am missing something.