How do you prove payment with regular Bitcoin or Dash?
An auditor audits it.
To be auditable, the auditor needs to see 2 accounts - not one. A debit in one and a credit in the other.
If I'm checking a payment I made, I go to the blockchain and see it leaving my account. I also see it arriving in the other account. If I only see it leaving my account then I haven't audited anything and if the other guy only sees it arriving in his account then he hasn't audited anything either.
In the fiat world, the auditor (as far as the transactional integrity goes) is the banking system itself, not some guy that walks into your office in a pin stripe suit and gets to look at your account.
In crypto, that counterparty has been eliminated, so Satoshi did 2 things:
[1] - broke the link between accounts and people so a blockchain address never corresponds to a person, it just corresponds to an anonymous private key instead
[2] - made the blockchain visible so that the entire world could serve as auditor and arbitror of its integrity
Take that away and you've got nothing.
What you've done is take a fiat business model (where every single peice of property of any significance is synonymous with a legal entity) and imposed it on a cryptocurrency (where it isn't). You've therefore given yourself a whole load of unnecessary problems to solve (which is why you need all that over burdening cryptography) while at the same time killing the very thing thats about 100 times more important than privacy - confidence.
With darksend you can't even do that. You have a coinjoin transaction (or a sequence of them if multiple rounds) and one/some of the inputs is/are yours but you can't prove those coins didn't go somewhere else.
Darksend simply improves the fungibility of the coins in your wallet. It's not trying to do what your doing (which is probably why your constantly criticising it). Moreover, it does so pre-emptively, exactly as real world cash does. At the point of transaction, everything is out in the open. It works exactly like bitcoin does. Privacy is already taken care of by the Satoshi model IF the blockchain is fungible enough (not invisible - different thing) which is why that challenge was addressed directly rather than indirectly.
As far as I can tell your problems mentioned are the problems of cash. You need some trust to use it. If you want a fully open ledger use bitcoin. If you want true privacy you have to pay the piper in risk. There is no in between. The thing trying to be in between will fail and be unobscured in the end.