multi-measure rest

You can already do this, just press "M" to toggle multimeasure rests - and like you said, if you do something wrong, you can kill it and try again. Also, if you want your rest broken into 3 then 2, this also happens automatically currently if you add a double bar, tempo change, rehearsal mark, etc - any of things that would normally *cause* a multimeasure to need to be broken. It should virtually never happen that you want to break a multimeasure rest for any reaosn that MuseScore doesn't handle automatically, but even if you wish to create that sort of non-standard notation you can get it by right-clicking the measure you wiush to break the rest and choose Measure Properties then "Break multimeasure rest". Again, though, you should never need to do this - multimeasdure rests are broken automatically at the "right" places by default.

Note that normally, one wouldn't be entering multimeasure rests at all. Normally, you enter a full score, and MsueScore creates multimeasure automatically for you when creating the parts. No need to count measures of rest - it does that for you to. I guess maybe for some reason you are trying to enter music that was *already* extracted into individual parts, rather than entering the full score? Thius is a very unusual situation, but even in such cases, it's still pretty easy to get the rest count right. Just click the first measure then hit Shift+right while counting. Or, just make a guess, then toggle the rests, see how close you got, then insert or delete measures as necessary.