Question/Problem 1; measure width

Staff size and music spacing settings don't change - they are pervasive for a score. but streetch settings can *override* the music spacing for some particular measures. It's possible you have very wide music spacing settings so new measures you add are wide, but you'd reduced stretch in measures 20-30 so those measures are narrower. Again, unless you attach the score - or even just a subject of it - all we can do is guess., It won't be a very efficient way of solving your problem. It might take many days of back and forth guessing before we hit on the solution. Where if you manage to attach the relevant portion of your score, I pretty much guarantee a solution will be posted within the hour. Your choice :-)

I'm not sure what document you started from, but nearly all measure in that score have some (rather extreme) stretch settings.
What I did was select all by pressing Ctrl+A. Then I went to Layout → Reset Stretch. Et voila, your 7 page score now became a normalsized 4 page score.

Do not change stretch settings of measures before you've entered all your notes into them; MuseScore does quite a decent job at making measures the correct with by default, depending on the contents of the width.
If for some reason you want fewer measures on a row, rather than using stretch (especially on empty measures); put in line-breaks. Once again MuseScore will then automatically adjust the stretch of the measures; but without going into crazy land as soon as you start filling out those measures with real content.

Indeed, it looks like your use of stretch is the cause of the problem. It seems in your experimentation you inadvertently added tons of unnecessary stretch and that's why things don't fit.

Measure 21 in particular was stretched 22 notches above the default - no wonder nothing else could fit on that line! To see the stretch applied to any given measure, right click it and go to Measure Properties. All measures start out at 1.0. Each press of "{" or "}" with the measure selected decrease/increases that value by 0.1. Measure 21 was at 3.2, whereas you shoyld probably never be outside the range of 0.5 - 1.5.

Really, you should not having to mess with stretch very often at all. If you find yourself doing this more more than very occasionally, chances are you are doing something wrong. Like increasing stretch where you should be adding line breaks, or decreasing it where you'd be better off switching to a smaller staff size or tighter music spacing globally. And normally, any changes to stretch should be for all the measures you wish to fit on a line, not for measures individually - otherwise you end up with inconsistent spacing within a line. Also, you should virtually never be increasing or decreasing stretch for individual measures - it should normally be for ranges of measures that you want to fit together. Otherwise the measure spacing will be uneven. Basically, the default spacing from MuseScore is generally very good and in accordance with standard engraving practice most of the time, and most people are better off leaving it alone. At most, you should be considering consider adding line breaks to force one or two measures more on a system, or reducing stretch across the whole line to get one or two measures fewer on a system, and if you're doing this for most than half the systems, you should instead be considering making global changes.

As it is, indeed, Ctrl+A then Layout / Reset Stretch is the way to put this score back how it should be.