The line improved last year - it's not a question. Any team can suck the coming season after excelling in the previous, but a statement like "the line is terrible outside of Kalil" is based on previous performance and is verifiably inaccurate. To your other point, EVERY team's pass blocking takes a hit when the defense knows they're going to pass and rushers don't have to worry about the run. That is not exclusive to the Panthers in any way, shape, or form and says little to nothing about the OL.
While a QB really selling a handoff doesn't hurt, defenses are reading their keys to determine run/pass. For the front 7, they're typically keying on a certain lineman or on a back, so that's where effective play action is sold. My guess is that comparing Carolina's OL vs other OLs on good play-action could be just as interesting as comparing QBs.
Per PFF, Norwell and Turner finished the season as the 16th and 23rd best guards in the NFL while Remmers was the top-rated tackle for several weeks. While not a definitive authority, those metrics give a good sense of performance and saying the line sucks outside of Kalil is incorrect.