No, it will not work anymore, not as is. The AppSwitcher example in the Kaleidoscope repo may be useful to see how to keep a key pressed nowadays.
As for one-shot double-tap sticky for a mouse key… hm. Should be doable with a thin, specialised plugin. On onKeyswitchEvent, it’d record the last pressed key. If that’s the left mouse button - again - flip a flag, and keep it pressed in beforeReportingState. If the flag is flipped, and the pressed key is the mouse button, unflip the flag.
That should be the gist of it, I think. If you need further help, please open a new thread, and I’ll try to help more.