I am not a real authority on this, I mostly just copy other code from examples already in the image.
I believe that if you are coding something that runs in the user local area and want to do something in the replicated space that "the right way" to do this is to send a signal to the space, or some other object in the space that you have a postcard for. That target object or space will need the proper event-receiving code to do the requested action. This is described in the Programming Guide section "Appendix I - Peer-to-peer Chat in Tweak". There are also examples in the SDK image.
As for written documentation of all the available methods and classes, this is a common complaint for Squeak and Croquet newbies (and I use the term with respect :) My advice is that you learn more about the Squeak development tools available, and explore the real power of the Squeak development environment. There is some documentation about these tools on the Squeak Swiki, currently at http://minnow.cc.gatech.edu/squeak/4 including the (Class) Browser and Method Finder.