Forgive me, Janie, if I suggest that some of your "problems" are unusual! You write as someone who has been around the singalong scene for quite a while. Unless you have a tendency [not unpraiseworthy---]to introduce new material often in your public appearances, it seems to me that songs with which you are long familiar shouldn't pose problems "key-wise". Most singers know from past experience which key suits their range in whatever song they sing. There are, of course , some factors which are beyond the control of the singer. One is encountered very often when required to sing along, or perform in unison with others. The comfortable range of YOUR voice can quite easily be a "no-no" with many others. Again, you may encounter musicians [not often, mind,] who cannot play in your chosen key, or who boorishly insist on playing in the key in which they themselves are most comfortable. The great majority of singers, however, don't seem to encounter such problems after they realise that it is a great help to musicians if they can be told the favoured key. Maybe you can find some comfort in the fact that this grizzled [truly}songster is compelled to sing down in his boots in order to accomodate the two female singers in our group. We all have a cross to bear!!
Boab