...In any case, my only point was that the use of a laser (like any collimation tool) requires careful consideration of the geometries involved. I daresay everyone involved in this conversation takes that as a given, so please consider the comment to have been directed at our silent audience
I'm still pondering the offset laser scenario.
It seems to me that such a laser, intentionally collimated with an internal tilt error to compensate for the offset, might appear to be collimated relative to the focuser axial signature (laser beam is centered in/on the primary mirror center spot) through rotation in the focuser, but will fail the primary mirror axial signature (the unBarlowed return beam coincides with the outgoing beam and falls inside the laser aperture) when rotated.
If the laser aperture is offset relative to the laser body by more than a hundredth of an inch or so, and the Barlowed accessory is centered on the laser aperture, I would expect the offset to still be relatively obvious. If, OTOH, the Barlow target is centered on an accessory (like the Blug or on an actual Barlow), I suspect the offset error will be significantly reduced.
Now, if the laser is offset and
the 2- to 1.25-inch adapter is also offset, and the two parts are allowed to rotate independently of each other in the focuser drawtube, I can't imagine a simple tilt error solution that will deliver consistent alignment of the outgoing beam or the return beam. A redundant axial tool would seem to be the best solution (and a replacement adapter!)
FTR, the OP's 4-inch f/15 refractor is likely to be quite forgiving when dealing with axial collimation. I usually collimate refractors with a windowed Cheshire eyepiece, verifying the stacked reflections through the focuser travel (I use a handheld fluorescent light to illuminate the Cheshire). I've always found this relatively simple alignment procedure to deliver excellent performance.