cross-signing & key backup are enabled by default, and have been for a year or so? you have to explicitly opt out by repeatedly hitting the 'skip' button. however, the UX is certainly more complicated than it should be, and we're due to rework it in the coming months.
Woops. I may be dealing with friends & family that joined prior to that. I wonder if one could get prompted again for enabling cross-signing & backup once you open a new client (I don't think it happens right now). That would help migrating everyone to enabling it.
