1. Exactly correct. You can set up multiple sub summary layout parts, each with a different break field and then you can "mix and match" by using different sort orders. You can even set up a sub summary layout part that serves the place of the report body and then you can make the "detail records" appear and disappear by sorting the records differently.
2. You really can't export layouts to do what you want. (It is possible, with a lot of "fiddly" to copy and paste layouts from one file to another, but that's not a practical option for what you want here.) But you can save periodic copies of your file and then you can "revert" a recent layout design change by closing your current file and opening a recent copy of the file. You can also use the Duplicate Layout menu option in the Layout menu to make a copy of your layout just before you try out a major redesign of it.
You may find the script for making periodic backup copies during the design process useful: Saving Sequential Back Ups During Development
Duplicate LO, great idea, wonder why I didn't see that. If I can put different sub-summary fields, I will need to place a button so the user can change the sort. Time to play around with scripts. Will give the dup Layout design a try.
I recommend the script. "Developer's Remorse" can hit for a wide variety of different changes--not just layout design changes.