Hello, Excalidraw+ cofounder here. The z-index syncing should be fixed now in Excalidraw+. The fix should make its way to the base app soon, after we're confident it works correctly.
The performance of the collaboration servers is now taking the front seat and should improve in the coming weeks.
As for the window alerts — yes, that's a pain point in both apps but we're working on it. It should be better than last time you tried it, let us know what you think.
We'd be happy if you file an issue or drop a message on our Discord channels with any other feedback you have!
They've been very good at communicating in the GitHub issues I've filed, so hopefully it works for others, but I would advise you to test with a few co-workers before facilitating anything in a group