I have bad news about who a convention of Biden loyalists will nominate regardless of what horrors Israel does. Or a committee of wise party elders (which doesn't exist), for that matter. The time to challenge Biden was before he won every primary with like 80% of the vote.
There's just not a realistic way that I can see where the convention nominates a progressive option. Even just replacing Biden with someone younger and the exact same policies would be nearly impossible.