For #1, just right-click the Month/Yr column header or data, go to Field Settings..., click Advanced..., and change the AutoSort options from Ascending to Descending. Click OK twice and you're set.
For #2, you just put in a formula to subtract one month's data from the next, unless you want to make it dynamic, where the user chooses which months to compare. That might take a bit more work.
Regards,
Ax