shmups.system11.org

Using the void bomb holds medal value though I believe, perhaps in said videos they were using that?

Silly question, but does the Steam version of Deltazeal offer any way to play as the 2P ship without having a 2nd controller plugged-in?

Yes, you have to hold X first (autofire button), then press A (shot) when selecting the stage/gamemode. The 360 version, which precedes the Steam port but shares the UI, has the same option as well.