Bit of brute force on Part II since I haven't learned iterative macros yet, but I got it and it is replicable. I have to admit, I am impressed with Caesar's command of the English language. The man was truly ahead of his time
I didn't feel like making a macro so I approached part 2 by parsing the string into individual characters, transposing this so that there is a single character per line, and then joining based on both the ROT-number and the actual character to be decoded. I then looked for "Caesar" to identify the correct results (as it was supposed to be signed).
Well, I don't think I'll ever forget lowercase chars are numbered 97-122 now.
What's great about this is it can encode/decode thousand-page documents in the same amount of time (or less) what Caesar's army would've spent decoding his 20-letter message.