Well, the key still matters if you want others to join in. If you just intend to play a solo melody, the key won't matter,but if you play something in B or G#, no one will join in.

I still don't think that even a loud uke will work well as a melody instrument at a mixed session. Each individual note is very brief, so you can't differentiate between a quaver and a minim except by the time gap in between. It won't be loud if other instruments that can sustain a note are also playing it. The loudness will only be effective with rhythmic strumming. Really and truly, a novice player needs to start with chords and learn how to fit them to a tune.

Marje