Right...........people saying that you're not compensating enough for squirt/deflection implies that you're just setting up wrong and it's not a mechanical/stroke issue.
You're saying it is a mechanical or stroke issue. I'd say most people can line up correctly but during the back-swing and forward stroke they pull themselves off the line somehow......sort of like when shooting a firearm when one jerks the trigger vs a gradual squeeze. It is very hard for someone to learn how to pull the cue back, have a smooth/controlled transition to the forward stroke, and then deliver the cue straight down the line to a distinct stopping point all while maintaining a perfect setup (especially with shots that require speed like you're talking about)..........head still, no unwanted shoulder/wrist deviations, no unwanted body movement up/down/sideways, etc.
Study/watch Darren Appleton videos.
If the OP has the issues to which you refer then why do they only appear for high inside english?