Paste copied notes by MERGING into measure- NOT replacing

I guess you mean inserting and the answer is that MuseScore doesn't support this. You need to cut and paste what you want moved over then copy and paste what you want inserted. If you want the measure to become larger then do this use measure properties before you start copying and pasting.

Please provide a precise example of what you mean by "merging".

There are different ways...
You can "merge" by imploding. See:
https://musescore.org/en/handbook/tools#implode

You can "merge' notes with different rhythms, see:
https://musescore.org/en/node/12345