I did invent a number of swear words and insults to suit the setting; however, writing is about communicating. Imaginary swear words never resonate with me as deeply as "real world" swearing does, and there are times I needed to provoke a gut reaction from the reader. I'm very careful with where I put the stronger cuss words, and try to use them as sparingly as possible. But "ta-karne" doesn't always convey the same effect as "you @**hole!"--which is, after all, what ta-karne means.
As for anachronism, not really. The concepts of "hell" and "damnation" are very much alive in the Northern Church; most southerners, unless they spend a lot of time up in the northern areas, don't use those terms. The southern religion does not assign sinners to a fiery fate. And in my view, a term such as "for unlawful carnal knowledge", under the Northern Church's strong conservatism, is a perfectly understandable development.
Finally, in some spots the story deals with mercenaries and sailors; and those are expected to swear heavily. Northerns, especially, wouldn't use the southern terms very often: so, as I deliberately patterned the common kingdom tongue after English, I had to use common English swear words.