2. Current disposable email requires a trusted intermediary
3. There are many tiny providers that might disappear overnight, therefore insufficient for using with financial accounts.
Apple offers something like this at scale, but only for Apple users.
What would a decentralized solution require?
The only method I know of that has even limited capabilities that you are looking for would be to register a few domains and point the MX records to a VM. Set up postfix to queue/relay the emails to wherever you want or to a local mail directory/file. Even that is becoming problematic with companies that have a large amount of fraud. The most notable I have run into recently was Wireless providers. I had to get on a call with their support team and escalate to unblock my personal domains. Even then it was case-by-case as my domain is only unblocked for my account, nobody else could use any of my domains without talking to support.