Keep in mind that using the largest measurement for both only works if your map is in the proper aspect ratio starting out. But if you make a square map (or any other map that doesn't have the correct aspect ratio compared to the paper aspect) that you just want to fit into the paper, you'll probably need to use the lesser of the 2 measurements instead, otherwise it won't be correct.

Being able to ignore one of the measurements and just let CC3+ handle it is a good and time-saving thing, but it does mean the map must meet the expectations.

CC3+ always does a split install. The actual program itself is always installed to the C-drive, but it is extremely tiny (less than 100MB with absolutely everything installed), the rest goes to the location you chose for your data directory in the CC3+ installer (Which I presume is e:\ something in your case), which is the bulk of the install (art, configuration, templates, maps, drawing tools, symbol catalogs, and all the rest)

Any add-ons installed after that will never ask for location, but just go to the appropriate directory of the two. Components needed by the program itself (mostly things like .dll files) will be added to the folder on C, but any content will be added to the data directory in your chosen location.

Not as far as I know.

But maybe you can work around it. One way I can think of is to combine all the individual polys into one massive one using the CONTOURSM command. This will create a single polygon that matches the complete outline of the existing ones, and you now have a single poly to use the area command on.

I don't remember the screens in the installer for the Annual 2023 off the top of my head, but at least some of our products have a screen with a checkbox that enable you to do an "artwork only" install (for use without CC3+). I don't remember if the 2023 annual had this, but if it does, make sure to NOT enable that option during install, or it will only install the artwork for use with other software, and not the CC3+ components. Try rerunning the installer and see if this is the case.

Some of the more typical things people forget are map title and other labeling, scale bar and compass roses, and they forget to use proper scale for the map. For the latter, when using Info -> Distance, the value should be what is expected. For example, if measuring along the a scale bar, the value in the dialog should match the displayed value on the scale bar. If the measurement come up wrong, it is likely that the entire map is scaled wrongly. Or if it comes up right, but the values on the scale bar doesn't match your intended scale for the map, it also means it is scaled wrong.