I am a Member of ( HAL ) Howard County Astronomical league. I'm the clubs GOTO guy for telescope repairs and upgrades.
I'm also on Cloudy nights (meademods) I sell clutch upgrades and other parts for the Meade LX200 GPS telescopes.
When changing to Bob's Knobs you remove one screw and replace one and so on (As you already know) If one screw appears to be loose and not tightening
you may need to loosen the other two and try again. The secondary mirror has (3) three springs behind it, that allows the mirror the be adjusted and possibly
one is slightly off center from the hole. Iv seen this happen before and you may need to push down on the mirror (the side the screw is not catching) and see if it will
thread in properly. Also most people do not know (As Meade will not disclose this information) But the Secondary Mirror "can be removed"
You will notice a small black powered set screw on the side of the secondary housing, Removing this screw will allow the secondary to be removed. (Very handy for cleaning the secondary)
Just make a note the mark the top collimation screw in relation with the top screw on the corrector plate.
Not you can safely perform the necessary adjustment's to get the last collimation screw in place.
Hope this helps.