how to understand those sentences

Joyce is, indeed, writing in a kind of code. Many of the words he uses are Anglo/Irish dialect, Latin or completely made up words. Understanding his language is not so much a matter of knowing what words mean as knowing what associations they conjure up. He uses language like a kind of music - each individual word may not mean anything (in a 'dictionary' sense) but the sounds of them make a constantly changing pattern of ideas and associations. I can only begin to help you interpret the passages you quote. A 'mutthering pot' incorporates ideas of a cooking pot (of some kind), muttering or mumbling (the sound that something simmering makes) - the spelling 'mutthering' reproduces the way that 'muttering' is spoken in Irish English - and, possibly, even the idea of 'mothering'. Gutenmorg is a purposely misspelt version of 'Gutenberg' and also reminds one of the German for 'good morning' - 'guten morgen' - perhaps giving the word an association of 'earliness' (which is reinforced by 'cromagnon charter'). The whole passage refers to the history of writing beginning with marks made on bones, pebbles and vellum (sheepskin) to Gutenberg's invention of printing (with references to 'great prime', 'rubric red', 'wordpress' and 'tintingfats' (which may either refer to the implements which early printers used to ink the type with or the ink itself, which is greasy). You may find this site very useful: http://finwake.com/