He's basically saying that he hasn't referred to the way he was received, or allowed the way he was received to influence what he has written.
He says that if he had done either of those things he would have failed to express the real depth of the appreciation he really felt toward the people he met.
He describes the people as having been generous towards him and not threatened him with guns.
In this sense 'sorry' means 'inadequate'
'Partial' means 'having a predilection for something', so they are fond of his books.
'Open handed' means 'Liberal and generous'
'Muzzle' means ' The mouth or the end for entrance or discharge of a gun, pistol etc.' though it does make on wonder how Dickens had been received in other places if it seems a generosity for people not to greet him with guns!