Ext5V is the best policy for using grid+monome modules from my point of view.
This will isolate the ground plane for the grid, i.e. no noise issues, and won't stress out your busboard either.
You can't remove noise that is being dumped onto the ground plane, you need to isolate it. Everything creates noise, some are just more noticeable than others.
Do you have the 1A +5V makenoise busboard version? If you only have the 500mA version, I would not power the grid with it.
Ground / Ground lift is fine anywhere really, it is better practice to have a mains ground though.
As an example, pretty much every single guitar pedal in the world is not grounded to mains earth.
Mains ground (earth) with any modular powered bus board is determined by the power brick powering it.
Typically an IEC 'kettle' style input to the brick will have mains earth connected. Although there are exceptions, Meanwell GSxxAxx for example. All Makenoise busboards are supplied with mains earth power bricks. Specifically for pressure points/rene as they need the mains earth to allow you to use the touchplates.