No it would not delay the Avanti - a Class 195 can reach Lancaster in 16 minutes from Preston. However, by following the Avanti, it provides a connection to Barrow from "The South". Also, there is no need to put the Manchester Airport - Barrow services onto the slow lines north of Balshaw Lane Junction -- a Class 195 could do 12-13 minutes from Wigan to Preston, and remain clear of the following Avanti service.
I don't know where you're getting the 12-13 minute figure from. The times I can see all show 16 minutes (+ 1/2 minues for dwell and departure). If you could direct me to the source, that would be great.
I slightly confused things by referring to 20:33/34 in the post you quoted when the train up for discussion was the 1C58 18:28 Manchester Airport to Barrow-in-Furness, which runs 1 hour earlier. That service does run on the fast lines.
This is my calculation:
1C58 (Northern service in question):
Preston - 19:34
Lancaster - 19:52
Carnforth North Junction (P) - 19:58
1S90 (Virgin Express):
Preston - 19:41
Lancaster - 19:56
Carnforth North Junction (P) - 20:00
There is a 2 minute gap between 1C58 departing Lancaster and 1S90. I'm not sure that the infrastructure allows that.
There is a 2 minute gap between 1C58 passing Carnforth North Junction and 1S90 doing the same. Again, I'm not sure that the infrastructure allows that. There is certainly no current point in the timetable that has services that close together at either location.