I googled for a pdf of the mixer you have, Mackie mix12fx with effects. It has a block diagram of the signal path.

The OP should be able to route a signal to the IEM's without need for a Y split cable.

The Main Mix outputs and Tape outputs are the same and and the output levels of both are controlled by the main mix Fader. The LED meters reflect the signal level for these shared outputs.

The Control Room outputs and headphones outputs are the same and the output levels are controlled by the CR/Phones level knob. The two switches for TAPE route either the Main Mix or Tape Input to the CR/Phones outputs determined by which switch is pressed.

All four outputs are the same mix (Main MIx)and is comprised of your channel fader position of each mono and stereo channel. The Main and tape outs levels are controlled by the Main Mix Fader. The CR Out and Phones 1/4" jacks are also Main Mix and the output level is determined by the CR/Phones knob. You only have level control of the Main Mix output between the two routing options but each option provides two connections. 1/4"-rca for main mix or 1/4"LR - 1/4" TRS for CR/Phones.

The Fx Send is a mono signal Post Fader of the four mono channels and a summed mono signal of the 4 stereo channels and are affected by the channel fader position, EQ settings, Low Cut Filter and the Gain Trim. Panning does not work on this signal. As with any AUX send, the amount of signal routed to the FX Send connection is determined by the FX knob for each individual channel. The internal FX's are not routed to the FX Send connection but are routed to the internal FX engine in parallel. To send a signal to the FX Send connection but not to Main Mix, turn off the internal FX by turning the FX to Main knob all the way off.

BIAB Ultra Pak+ 2024:RB 2024, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.