[Darley]

Taking apart a lens isn't that bad. (well, primes anyway, I'm not too confident with a zoom yet)

It's also a good way to be able to score dirt cheap lenses, I bought a Pentax-A 28mm F2.8 with some pretty hideous fungus for cheap in a lot of gear (I sold some of it and made most of the money back)
Some disassembly and some isopropyl alcohol and I had a mostly fungus free, fairly good, lens again.

Yes primes are definitely a safer bet for the novice. I wouldn't fancy trying it on a modern AF though, especially an IS lens. Not unless it was free

[Darley]

The distance is the same, Canon's mount is a few mm closer to the sensor than Pentax, the adapter compensates for this because the registration distance is determined by the lens. At the same focus distance the rear of the lens is exactly the same distance away from the sensor on any camera.

I think it was just luck, an out of alignment lens will be bad no matter what it is on, for some reason the alignment was temporarily better. Could have even been down to the way you were holding it at that moment.

One thing I considered was the lens fits the Canon slightly rotated (the aperture marks are a few degrees further clockwise) so if the lens was out on one side maybe this compensated for it?