What variations are there on Solo whist?
The Pagat site is very useful:
http://www.pagat.com/boston/solowhist.html

It explains the main game "Solo Whist", and also lists some minor variations, and other similar games in this "Boston group".

They say that it is mainly played in Britain. Here in Australia we also play it. Solo Whist was taught to us as children to introduce us to card games, and has held us in good stead for Bridge.

Regarding the "Prop and Cop": We believe that that is an important aspect of the main game.  We do have that bid, and we find it to be very enjoyable. It is an opportunity to collaborate as two teams, rather than three people collaborating to defeat the solo declarer. If a Prop-and-Cop does not eventuate, then we also sometimes do the "Goulash" deal.