[Hikikomori]

For your second scenario you should announce the /23 to both and each /24 to one of them. Usually you can also prepend your own AS, ISPs I've worked could also prepend for you with select communities.

I don't think your cases are good enough to allow anyone to spoof by default.

[Habgdnv]

I said that we tried this. They do not care about the announce. They care about what is injected into the routing table. We are announcing it but they see better path and drop us.

And they also said that it should work that way - just announce it somehow and it will work. Yes but no. It does not work.