Guys - I'm very confused...
I have a good friend who has been a LONG time Mach1 owner - is incredibly knowldgable on all things (has written his OWN imaging software equivalent to SGP or NINA) - and according to him:
"I've always had the 'Reverse Dec output after meridian flip' enabled. I think that is all there is to it. You need the mount to report the pier side properly so PHD2 will know it needs to reverse DEC. I neer had a problem with the Mach1 doing this. I did use the meridian delay feature you are talking about some as well.
If you uncheck the reverse dec box it will certainly not work. "
His advice to me on MANY, MANY issues has NEVER been wrong, so I am really struggling with you all saying he is wrong...
Seems like this should be a clear un-ambiguous answer... I guess from you guy's perspective it is :-)
I will experiment the next time out - try that testing software.