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i might be well off the mark here but its something that may need to be thought of.
I have recently had a head smashing moment where no matter what i used to get colmination i could never ever get perfect focus (accross the field stars where not the same)(i used all sorts of methods, laser, barlow laser, cheshire and all high end equipment none of the gso varients) it ended up being that the tilt of the actual focuser was not inline with the ota axis (it is a newt and yours isnt) but even with your colmination being spot on you focuser could be slightly and i mean slightly tilted in which case your FF will magnify the problem as the mpcc magnified my focuser tilt problem.
how did i check? well quite simply actually ditch your laser and grab a cheshire that has a silvered cutout look at the rings/lines/circles that is being reflected back if you can see nice sharp well defined and concentric rings/lines/circles that means your all good remembering to move focus in and out during this as thats how your reflected image will differ! If it does differ, look at the tilt on your focuser then once everything looks the goods recolminate your scope.
Now i get multi coloured diffraction spikes and virtually spot on star images when using stars to collminate! furthermore the inside defocused star is virtually identical to the outside defocused star meaning that my tilt is now eliminated or reduced to a tolerable level.!
Like i said it could be truely useless/ful information but anything is worth a shot when your at stumps end.!
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