rule checking when entering or editing notes

...rests being added then no space left.

Such rests are not written in concrete and so can be easily replaced with notes "however you want".

In fact, a blank score initially displays full measure rests throughout. It is expected that these will be overwritten by notes (and other rests).

Remember: 0 (zero) enters a rest of a selected duration, and so are entered similar to notes.

In the default step-time note entry method, you'd indeed overwrite whatever was already existing. You provide a time signature and MuseScore provides you with measures in accordance to that time signature. When entering your music from left-to-right this should all turn out perfectly (definitely when copying a perfect score ;-) ).

Why can't you enter all the notes however you want and then have the program check it and point out any errors like a word processor
You kind of can; by switching to insert mode. Then everything you added will extend the measure duration by what you've inserted. Of course then afterwards your left with a lot of cut-and-pasting or splitting measures to once again end up with the desired score and have to correct lots more.

Have another look at the "Basic note entry" part of https://musescore.org/en/handbook/3/note-input.
Keep in mind to always enter your music from left to right, including the rests. Then feel free to post an example of what you're copying that's failing or where you feel these instructions don't work out as expected.

Unless your score contains errors (so it has more or fewer beats in a measure than the time signature indicates) then nothing about how MuseScore keeps your measures consistent should get in the way. probably there is something about the process that you aren't quite understanding yet, but it isn't clear what. If you attach a sample score and describe a particular task (entering particular notes into a particular measure) you are confused as to how to accomplish, we can help you see how to do it simply.