The concise reply:
1. Liquidity [look it up] is only possible in an economy that has cash, but media of exchange emerge in barter economies [which have no cash] as well.
2. You showed that wide demand does not imply marketable. But I am claiming marketable implies wide demand. Not the same thing. Just the opposite, in fact.
3. Menger’s definition of “saleable” is not relevant to Mises when he talks about the regression theorem. Mises is talking about a barter economy [as well as a cash economy]. In barter economies, there is no cash price for things, because there is no cash.