Printing and slicing big pieces with Elegoo Mars (Pro)
With so many questions, I'll provide a suggestion for your first one, the alignment option. There's a 12 minute YouTube video showing how to use Meshmixer (free, multi-platform) to create alignment pins and holes while segmenting a model for printing large items.

The process involves making the cut, keeping both sections. A pin is created and sized, then placed in one of the faces, while the other segment visibility is turned off. The pin is then triplicated, with one pin moved away for printing.
One of the two remaining pins (overlapped) is selected and is used for a Boolean subtract operation, creating a hole that is matching in size and location.
The first segment visibility is turned off, the second segment is turned on and the pin is selected again. The sequence is duplicated, creating a second, matching hole.
I didn't watch the entire video, but if it is not suggested, one should reduce the third pin size, at least the diameter, sufficiently to allow it to fit the holes created by the above process.
Note that the video has some inconsistencies from my experience. Some terms are changed from the video to the program, but should not cause problems.