Marx tended to focus on roles more than persons or transactions. Buyers and sellers, capitalists and workers, etc. Any individual might play any particular role at any given time. (For instance, I personally fill the role of worker with my day job and capitalist with my 401k.)
The reason I mention that is because his thesis was that, in aggregate, the role of capitalist must expect a positive return on MCM transactions, or they wouldn't occur. Individuals may suffer losses, but as a role it just be positive.
He's less data driven than Piketty, and he builds this notion on top of the idea that there is a desire to hold wealth. Or, put another way, it is natural in an economy of commodities and money to prefer holding money over any given commodity. Eg, having a hoard of money is more useful than having a hoard of wool, as it is generally easier to convert money into a commodity you need than to convert between two commodities.
So the natural inclination is to try and increase wealth, and MCM exists commonly, so therefore MCM must be a net engine for increasing wealth. (Even if individual cases are losses.)
I'm less confident in answering your first question. It hasn't come up in my reading, but I'm not a scholar. It could be in there. (One of the things Marx often does is open windows to look at something and then move on without diving deep on them. For instance he acknowledges inflation, and moves on rather quickly.)
No problem! Again, I'm just some internet person who happened to listen to some lectures and did some reading, so my take might be a bit off. But I find this stuff fascinating. Doubly so for something like Marx who everyone has an opinion on, but I feel very few folks have actually spent much time diving into.
Did Marx address the notion that collection of rents on capital create an incentive for the creation of more capital?
How did Marx address the case of MCM transactions where the capitalist has erred and lost money?