Using the Y-Splitter is not ideal for this configuration. Why not plug the guitar directly into one of the channels mixer instead of plugging it into the MixingLink?
Alternatively, you could try:
Mixer FX SEND -> ML MIC/LINE in
Guitar -> ML INST IN
ML DI/LINE OUT -> Mixer FX RETURN
The input gain will apply to both the MIC/LINE in and INST IN simultaneously so you’ll have to set the level of your sources accordingly, but this would be a better configuration than using a splitter to sum 2 sources to the INST IN.