> USB stack(poseidon) conflicts with MIAMI: so you can't use it while your online.
You're the first reporting this -- I've been online while developing every day with Miami(Dx) and had no problems with Poseidon at the same time.
Concerning the Boot menu: If you have a flashrom system or otherwise make Poseidon reset resident, you will be able to use the USB mouse in the boot menu aswell (but this only applies to host controller cards from E3B (Algor, Highway, Subway), because Elbox drivers cannot made reset resident for boot time).
If you want to play games (WHDLoad), you're better off with something (e.g. Cocolino) that plugs into the standard mouse port.