We did a very brief trial on cross-coupling the Approach North and South frequencies, I should know, the incident that Point 4 refers to was me!
It is a design function of the Radio kit (Schmid) that we use, that involves micro-seconds and all sorts of tels type boffin stuff, that the aircraft who responded quite clearly on 118.57 to the call intended for the aircraft on 119.4, was not heard by either me or the other aircraft because it was never re-transmited.
This happened because his transmission started after and finished before the readback on 119.4, the system waited to re-transmit, then there was nothing to re-transmit so it did nothing! There was no squeal to alert me that there had been another transmission made. Both aircraft commenced their descent and good fortune meant there was no loss of separation.
We do still cross-couple multiple (3 x VHF and 2 x UHF) Tower frequencies during the quiet night periods. This cross-couple problem does still occur but all the traffic that is in the air or on a runway must be on 118.62.