Ortho doesn't like when the snap grid is a hex grid. Right click ortho, and set the grid to any rectangular grid instead of that "user hexgrid", that should fix it.

Correct aspect ratio compared to the paper size you exporting to. For example, if you have a 1000x800 map, your aspect ratio is 10:8, but a standard A-size paper is closer to 10:7. So if you just blindly type in 297 mm for both dimensions, the exported image will be 297 by 237.6 mm, which is larger than the 297x210 mm of an A4 paper. (I didn't account for margins here)

CC3+ automatically calculates the other dimension based on the aspect ratio of your map, not the paper.

Of course, if you don't mind it getting a bit large and simply resize it down to fit the paper on print, then using the largest value works, but if you want it accurately, then you have to actually make your map in the right aspect ratio from the beginning, or you need to also calculate the lesser value so CC3+ can export an image that fits within both the dimensions.

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.

These maps are now in the atlas. Great work. Thanks for the contribution @Don Anderson Jr.

No, CA3 doesn't really care about the location, it just lives in the CC3+ folders wherever they might be.

Did you get the right version of the CA3 installer? There is one installer for CC3 and one for CC3+.