This maybe sounds stupid but hey, anyway.
I am relaying a few stations using Mountpoint relaying not master relaying.

There is a chance for this mountpoint to fail from the other side (other server not mine).

What I can do for fallback except autoDJ ? It't not enabled so I had to delete/re-create a dozen accounts.
Can I somehow have another mountpoint relaying as fallback ? Can icecast configuration "allow" two <relay> options ?
So when the first remote relaying fails, it will switch to secondary fallback and connect back to first when it's up.

Is it doable ?

If not, propose a proper way to fallback to something when the relay is off.
I don't want AutoDJ except if there is no other choice. liquidsoap/icescc consumes a lot of CPU. No sense to consume CPU for backup only.

this is an interesting subject and my first thought is to write a simple script that would look for the first stream and if that failed look for the second stream. How to do that is beyond my skills although if you are interested I'd be willing to help you find the answer.

Like 440music says. Do an Script where the Visitors where routed. On your Icecast Servers you can use the fallback part to route users to another location on errors. Where using those setups without problem for high frequented stations which needs an 100% uptime guarantee.