If it isn't in the contract, then it isn't enforceable at all.
If they are being honest, then it is not real drama for them to sign what they're being honest about. If they're not, however, being honest...
It somewhat sounds like they're hedging their bets - if you don't sell well, well, bad luck, the first 15 are on the house. And if you do sell well, oh, gosh, remember that commission structure we talked about on the phone? The one where you earn less, and not the one we signed? We're using that one.
Sales is tricky, and if you're experienced I wouldn't sign a contract that was so obviously untethered to what was verbally discussed. However, only you really know what you want - I'm putting this in because you also seem happy with the potential of this contract to hedge - but you're simply looking at it from the other POV. (if I'm bad I discount the contract, if I'm good I argue to enforce it).
I'm not sure schrodinger's contract is the best way to work with other people, but this is all up to you.