maintain a queue of.. failed modifications and retry them later when the API goes back up again.
Your blockchain client still has to have a queue of transactions to be submitted, as the Internet is not magically reliable.
You're literally describing a benefit of blockchain. Its right under your nose.
It is a benefit of the blockchain, in the same way that transporting people is a benefit of a dump truck. That doesn't mean it's appropriate for that job.
There are many, many tools for storing and pushing messages to remote servers. The blockchain clients can do it, but it's absurd to use them unless you need the rest of the blockchain stuff. Even an email server (MTA) would be a better option.
maintain a queue of.. failed modifications and retry them later when the API goes back up again.
neither would I have to
provide the software that does this to the client so the client doesn't have to do any work for it.
You're literally describing a benefit of blockchain. Its right under your nose.