Channel weighting of Mono channel in EBU R128
Use a channel weight of 1.0 for your mono signal.

The weighting of 1.5 presented in ITU-R BS.1770 addresses the fact that a signal coming from behind is perceived louder by a human than if it is coming from the front (Reason: To better detect an enemy sneaking at you from behind). With a single mono signal this fact doesn't need to be addressed.

The 3 LU difference you mentioned also makes perfect sense from a physical point of view. One speaker in front of you compared to two (equal) speakers only provides half the energy and therefore the SPL will be lower by 3dB.

I'm honored that you use my LUFS Meter to compare to, but I'd recommend to also use the test files from ITU and EBU. The LUFS Meter manual contains the links to these test files.

Are you aware that the sources of the measurement algorithm of the LUFS Meter as well as some development notes are available? (Sadly I'm not allowed to post a third link... They are at docs/developmentNotes/)

Disclaimer: I'm the developer at Klangfreund.