You got the fueling setup for it, so no reason to bother blending. Save the headache.
The rule of thumb I've heard is R/S3, stick with oem LC\TC. GTI, go with Cobb. So no, don't bother with it.
When you scan your car, if you currently have the P0420 cat efficiency code (if your downstream O2 is plugged in), or the P0036 code from having the downstream O2 unplugged, then yes you will have the CEL with the new tune. If you don't have either of those codes in the background, then theoretically, you shouldn't have the CEL with the new tune. Either way, if you swap back to your old 93 map, it'll be the same as it is now. No CEL either way.