CAAD12 - cables 'sticking' in frame, worn groove?

Hi folks. Just came across this thread when searching for CAAD12 rear derailleur cable replacement.
Shifting poor on my rear derailleur. Cable needs to be changed. But see the attached photo. I gave the bike to a mechanic a few months ago to replace front derailleur cable. It’s looks like the cable guide cover wasn’t replaced correctly (there is a gap between the cable guide and the frame) and when I replace it correctly after installing new RD cable it’ll change the tension of FD cable and so will have to also set up FD again? Am I correct with this assumption?

Also, when I’m replacing the rear brake cable on this bike I guide an old cable liner over the old inner cable through the frame so I can easily guide the new inner through the frame (if that makes sense). Is it possible to do the same for the rear derailleur cable?? Is it possible to use a full length cable guide from top of downtube to end of chainstay?
Thanks!