With an instrument or with a choir what are the main concepts that aid a lead vocalist in staying in key
The most important thing is probably to be able to listen to what's going on around you while you are also singing. If you are singing with an unaccompanied chorus and they start going flat, you don't want to stay in key; you want to adjust to them.
Knowing how to adjust your pitch slightly higher or lower depending on the melody and the harmony, or for an expressive reason, is important.
Having a good vocal technique so your voice is actually producing the pitch your ear wants it to produce is another important factor. Many amateur singers simply lack the control needed to sing the pitches they want to sing; caricatures of amateur "off key" singers typically capitalize on that.
As Tim says, key and tonality are actually secondary here. What matters most is whether the pitch you're singing at any given time matches with the pitches others are singing or playing at that time. If a piece opens with C and E flat, it doesn't matter whether the key is C minor or A flat major or anything else. What matters is that everyone with C matches each other and everyone with E flat matches each other and that the distance between C and E flat is the correct distance (whatever is "correct" for that piece, depending on the tuning system or temperament chosen by the performers or the music director).