Measure changes time when I delete a rest or note.

” Sometimes two 1/4 rests that I’d like to make into a 1/2 rest. I’m usually unsuccessful.”
Try this: select the first rest, then press the correct shortcut key to change its duration (5=quarter note, 6= half note). To put it another way: do not delete the rest, instead increase the duration of the earlier rest.

” So I’ll delete one of the rests and a little grey box appears above the measure indicating the time of the measure is not the 4/4 timing I started with.”
It sounds as if you are deleting the rest with Ctrl+Delete which does indeed remove that duration from the total duration of the measure. To avoid this, you can delete rests in Voices 2, 3 and 4 with a plain Delete. But you can never delete a rest in Voice 1- instead you can make it invisible.