Firefox

put a big banner for iOS users telling them that apple doesn't let you test it, and that any complaints should be forwarded to apple

Didn't even think about that haha.

I guess the best you could realistically do would be to adhere to web standards (not Chrome standards) and use desktop Firefox or Firefox on Android for testing as they should be the same internally as the hypothetical iOS port.

You don't and that's a feature for Apple.

Same way you test on Safari if you don't have a Mac, I guess. (i.e. not at all, or with the same rendering engine on a different device and hoping it is similar enough, or via a service like Browserstack.)

It'll fall on deaf ears but complain as an ios dev to apple

Pray it just works? Get consumer-friendly legislation to pass in the US somehow? Maybe a genie wish or an infinity gauntlet could be used for this purpose.

Apple has never been great at enabling developer testing. I certainly don't see why they'd care if shit works on third party browsers. The more broken apps are just means the more users who will give up and use Safari.