Yesterday, I spent time thinking through each of the issues that you raised. As I worked through each point, I realized that I am making a lot of assumptions that really need to be clearly detailed. There really needs to be a technical draft with my assumptions that can be referenced in my reply.
I realized that perhaps the most nonstandard feature of my approach are my assumptions behind a unique, identifiable "coin" which is a vehicle for saving or investing versus a "token" which is really a credit that is used for "spending". A user would buy "tokens" directly with the assumption that if the token is not used, it can be returned to get one's money back. So, a token, in this sense, is like a postage stamp. You will probably need more tokens as prices go up but the tokens themselves, in theory, should not change value too drastically.
As I write this, I realize that this distinction between "coin" and "token" is very much based on a set of rules that at this point are only in my head. So, the next step is to detail my idea about this distinction in a white paper and also in reference code to show this distinctions I am assuming so that others can kick the tires and make sure that the car can can actually run at least on test road.
My next step is to write draft 0.1 of a paper which will focus primarily on two topics: 1) Accountable Blockchain: a proposal for offering buyer and seller protections. 2) Protected Coin: a proposal for distinguishing "savings" versus "spending" by distinguishing a unique, identifiable, mineable, "coin" from a quantified, account-based, credit which is used in transactions.
With the idea that more information will be coming soon, I will attempt to address each of your points.
The ability to change private key does not require that the private key is backed up (how the private is secured is handled by users independent of this proposal). As long as a blockchain timestamp and a public key are stored in the blockchain, that should be enough. When a user validates their identity through multi-factored authentication, they can invalidate the previous public key for all future transactions and can register a new public key to be used.
The ability to reverse a transaction becomes possible when the assumptions about "coin" versus "token" are better explained. In my proposal, a "coin" would not be transferred through 10 transactions. Only a "credit" would be transferred so that each transaction could be reversed independently of the others. The reversal would mean that any digital good would go back to the seller and any "credit" spent would be recovered by the buyer. This protection is not perfect and is not a guarantee. It has limits which would be specified in the contract (each transaction would be qualified by a smart contract). The constraints which are negotiable and the protections for this reversibility would depend on a specified escrow period (before the transaction completes) or a margin (which is the amount that a seller agrees not to spend in order to provide the reversibility guarantee). I have some ideas on how the system can encourage escrows and margins which I will detail in a future draft of the paper.
The willingness to reveal identity is meant as the final escalation in the dispute system if the dispute has not yet been resolved by the agreed upon contract. No one would be forced to reveal any private information and it is assumed that in most cases, there is no need to dispute a transaction. My assumption here is that the rules of dispute would be specified in the contract when the potential buyer sets up their "spending" account and will be part of the "offer" that the potential seller sets up in a marketplace. If a person wishes to accuse another person of a "crime" and feels that the dispute system has failed because of "fraud" or "theft", then they would have a process available to reveal information about their identity. If the other person (buyer or seller) wishes, they can block this escalation by revealing additional information about themselves. The idea here is not to limit anyone but rather to provide a protection against scams, frauds, and theft. Once an identity is revealed, standard legal channels can be appealed to. I am assuming that a scammers, fraudsters, and thieves will prefer to reverse the transaction rather than reveal their identity and I am further assuming that this process of revealing identity in a public way would be rarely done rather than commonly done.