Interim steps like you suggest don't really change anything.  What matters is the total number of transactions processed between the time Black Bart spends his coin and the point at which you (unrevokably) transfer something to him in return.  (Called "confirmations" in the current UI.)

If Black Bart controls fewer CPU resources than the swarm, increasing the total number of confirmations (again, regardless of the number of interim transactions) dramatically decreases the probability that Black Bart can "change history" by providing an alternate block chain.

However, if Black Bart controls more CPU resources than the swarm, all bets are off.  He can change history pretty much at any point he chooses.

I'll reiterate however, that the cost of dedicating more CPU resources than the swarm to ripping off BTCs is virtually guaranteed to be many times greater than any amount of value you could extract from the swarm.