sends/receives rip updates via broadcast as default
Its classfull no matter if you disable auto- summerization which means ripv1 will not send routes with its subnet mask in its updates unless that route matches the classful subnet mask of its interface.
sends/receives version 2 updates via multicast but can be configured to use broadcast and send/receive ripv1 updates.
It is also classful by default but if you disable auto-summerization it becomes classless, meaning even though you can still can only apply classful network statements to the routing stanza, It will recognize and advertise the correct classless subnet.
So by default ripv1-ripv2 not compatible.but they can be configured to be so even when one rtr is running ripv1 and the other is running ripv2.
Lastly the reason why communication works when you changed one router from version2 back to version1 was the routes in the route table hadn't timed/flushed out but when you manually cleared the route table this caused communication to cease and routes that was learned to be removed.