There is no evidence to suggest that Coverdale's confession, which is so dramatically revealed in the final chapter of the book, is something which could be doubted. Rather, in a sense, it beings to cast light on other events in the novel and it helps the reader understand why Coverdale remained part of the social experiment for so long when he had very little else to keep him involved. In addition, something else that seems to support the truth of Coverdale's words is the way that he has lived his life after leaving Blithedale. Note how he explains his life in the following quote:
My subsequent life has passed,—I was going to say happily, but, at all events, tolerably enough. I am now at middle age, well, well, a step or two beyond the midmost point, and I care not a fig who knows it!—a bachelor, with no very decided purpose of ever being otherwise.
The way that he describes his life as having been led "tolerably enough," deliberately choosing to use these words rather than "happily" indicates that he has played the part of a man who has lost in the game of love and whose life has been blighted by unhappiness as a result. The way that he declares himself a bachelor and has no intention of ever marrying likewise supports his continued love of Priscilla: having been in love with her, he finds it impossible to entertain ever marrying, as clearly no other person would be able to compete. There seems to be no grounds for suspecting that Coverdale is being false in his protestation of love for Priscilla; rather, it helps explain a number of aspects about his character.
You say there are no grounds for doubt, but the previous 28 chapters provide no evidence of Miles' feelings for Priscilla. He seems much more attached to Hollingsworth and it is him he feels lost over. He remains a bachelor because he cannot marry Hollingsworth. The evidence you give only shows that Miles is in love with someone, not unquestionably Priscilla