Try uninstall mix control.
Reboot
Install latest.
Reboot.
Set to defaults everywhere in mix control. Set it up again and save to hardware.

Don't do ASIO4ALL ever...

The easiest way is to not monitor in software. Set up a small mixer, and use the aux to route a wet signal to the vocalist. You'll get better timing, a better performance, and save yourself work.
 

Nevertheless it show be extremely low in terms of latency. The other thing is to get out a copy of latencymon.

It is worth saying you should not be listening to the DAW and mix control at the same time. You will get a slight echo otherwise, as I see no zero latency feature for this interface. I would just monitor through the DAW and mute mix control.

You might try a loopback test as well to optimize your offset.

This tutorial is the best I have seen talking about MixControl. It is 15 minutes, but worth the time.
 
Short answer to your question is, yes, you can do this. In MixControl the label at the top of each fader is what the input is to that channel (Anlg 1 is dry, FX (Anlg 1) has compression/EQ baked if used). In MixControl, you can send whatever (Mix 1) to any monitor point. These all are done by clicking the "Input FX" button in the middle left and selecting "Router."
 
For what you are doing, you want the mix set up to listen to the DAW, and add processing to the rest of the mix. To do this, do not echo the armed track in SONAR, in fact, mute it - that counts as a "DAW" signal and you do not want to hear it twice. Now your mix has DAW output + FX on input -> Send "Mix 1" to monitor/output of your choice.
 

1. Be sure the top of the first faders read "Anlg 1" and "Anlg 2" (this is going to SONAR, dry signals)
2. Mute those inputs on the armed tracks inside SONAR (to prevent slap back echo)
3. Tailor mix/FX/reverb, send Mix 1 to monitors

The recorded signal will be dry.
 
   Also important, on the "Reverb Send" tab, do not send the DAW output to it (only Anlg 1/2)... otherwise you will hear reverb thinking that track is wet, when it is not.
 
   Another quick way to check this is to select "Routing Preset" -> DAW Tracking.
 
   Once you have configured the above, File->Save to Hardware (I have a Saffire, and "think (hope?)" this is on the Scarletts as well).
 
   From that point, you will only need to use the "Routing Preset" button to do this again.
       "Routing Preset" -> Ultra Low Latency Tracking (assigns Mix 1 set above to outputs)
       "Routing Preset" -> DAW Tracking (listens to the DAW only and sends to outputs)
 
   You can get more complex than this, but that is the crux of it.