Recover deleted files from Filevault 2 On one partition of an external drive encrypted with Filevault 2, I have accidentally deleted a folder and files. Does decryption also decrypt the deleted files?
No - encryption/decryption is a completely separate process that will not affect what the OS sees as data (files/folders) versus available space.

You will need to run a file recovery program, such as DiskDrill or Data Rescue; the chance of success at retrieving your files will vary.

Finally, you should seriously consider a proper backup solution, anything from Apple's built-in Time Machine to 3rd party solutions like CrashPlan or BackBlaze.