Couple issues in this post. First, yes, the default action (double-click) of some things was changed. We're continuing to think on whether whether further refinement (or possibly configurability) would be good here. Where the command you want isn't the default action, it should be in the RMB.
Second, the ability to 'build' a repeat region callout, selecting asm... then mbr... then name, for example, still exists and works as it always has. This works only in repeat regions, and this is also as it has always been. There has never been a mechanism to, say, 'insert a parameter callout in a note by selecting the part, then picking the callout from a dialog'. No functionality was removed. If you think something along this lines would be useful to your work, a product idea / enhancement request would be the way to go.
Third would be how to get a combined quantity for multiple assemblies. There is unfortunately no syntax to reference a value computed in a particular repeat region record. One way would be to make a wrapper assembly, not employed in any view, which contains both A and B. Its purpose would be to drive a repeat region which would call out the quantitty. The drawing would then have views of A and B (and many tables), and another table using the wrapper assembly. This would be able to cover any number of parts.
Last, all available existing parameters is an interesting concept. You can see all the parameters for a particular context by going to the parameters dialog. However, if you mean 'all things that can be parsed into callouts', from &weight to &d4 to &g3 to &total_sheets etc., there is no place where they are all gathered together.
