Hello Chris,
1) It is hard to be sure, but I have never experienced impact on performance from sometimes huge amount of "monitors". And I was using slow computers in comparison to modern systems. If you have some doubts, use "Dup" button in Options to duplicate controls with monitors. Try 10,20,100... till you observe problems. But do not be surprised you need more then 1000 monitors till you notice any difference.
If you still worry, set the "Speed" to something slower then "Ultra". Rings and buttons are good candidates. That divide the number of calls by 2,3,etc. That parameter was introduced for that purpose, but there was no evidence I ever need it.
Just an example. With AZ Controller GUI open, I monitor all parameters for current ACT plug-in. Advanced synths have 2000-3000 of them. Do you notice any performance problems when the GUI is open?
2) monitors (except state and "once" timers) are checked every time (using Speed specified). But you can "void" monitor by some "final" action before Monitor in the list. If such action is the first (obviously make sense with some condition only), almost no code is executed.
3) it is good idea to always have ONE monitor which sends feedback to particular control. Other solutions is looking for troubles. So, if encoder E1 can control pan or input gain, the selection of the parameter to control should happened before single Value monitor. That way the monitor will update the ring when the value is changed OR parameter is changed.
For that purpose in most complicated cases, when one control can perform completely different operations (f.e. control pan or jog) and so require different monitor types in different modes, there are "state dependent" monitors which change the type based on current software state.
In general, even "Ultra" monitor is executed once per 75ms, in not critical thread. Any "audio processor" has to process at least 44 samples per 1ms in real time.
MCU has more then 100 controls, all with feedback. And people used MCU+2xXT+C4, so like 500 feedback parameters, on single CPU computers more then 25 years ago.