Well, if thats what your seeing (slight shifts in the central obstruction) This has to be the problem I would suggest.
The 3 collimating screws are tricky too. All 3 must have the same (or very close to) the same amount of tension on them by the time you have FINISHED getting your collimation spot on. An example: I had collimation spot on, but 1 screw was tight, 1 screw had medium tension, & the 3rd was only finger tight. All that = no good! Pinched optics & slight secondary movement.
Not having your SCT at your place today is a good thing! Can't have you jumping in boots & all without letting you know all this!
Correct, no pulling the corrector out to do any of this.