Can I mix joint compounds with different setting times to get a custom setting time?
The short answer is yes.  I have done this but not for the reasons you have stated.  I just had some leftovers and wanted to use them.  As long as your mixing agent - water - is the same then mix away.  Note that other than trial and error you are looking at a setting time between 45 mins to 210 mins - even if you mixed 50/50.