HI Paul --

Registering any version of a UserActions script will replace the previous.
Yes.

If one wished to incorporate features such as this into an exisiting modified version of UserActions, one would have to merge the code?
Yes, only one UserActions can be active at a time.

could #waituntil be used as a rudimentary way to add some 'timing' to each target in the plan?
Yes, that is the purpose of the #waitxxxx directives:

As always, this is a roadmap to the info not an "RTFM" criticism :-)