In the attached, you can see that I am trying trill from a Bb (in the existing key signature of Bb) to B nat.
As far as I know, using the properties to have the trill move up a minor 2nd (ie, up 1/2 step) to B nat creates the accidental of “b”, which of course is harmonically correct to Cb…ie, B nat.
How do I get the accidental to read a natural but still play back Bb to B nat?
Hi. I think if you want to be understood perfectly, you should use the version of the trill where the note you trill to is in parens, cue size. And the interval you should use is augmented unison, not minor second (which is Cb) if you want to show a B nat.
Given your notation of the run in bar 269, I think the trill would be clearer as A# trilling to B nat (That would give your desired nat sign above the trill).
You could then choose whether to continue with A# or do the enharmonic change to Bb.