For the irregular stems in ρ, the book I'm using states for the irregular stems in ρ:
"Elsewhere the accent of these three words falls on the stem-ending -ερ- (or -ρα- in dat. pl.) or on the case ending (in the gen. and dat. sing., where the rule about monosyllabic stems applies normally to μήτηρ and πατὴρ, while θυγάτηρ is treated similarly by analogy)
Is the book saying that μήτηρ and πατὴρ have monosyllabic stems? But isn't the stem disyllabic? μή-τηρ?
My second question is, what does it mean when it says 'by analogy' for θυγάτηρ? I don't understand why it has an accent on the ultima, like all the other monosyllabic stemmed words, when it is clearly a trisyllabic word.
My third question. All the other monosyllabic stem consonant declension nouns, like ἅλς, ἁλός keep to the rule (accent on U if monosyllabic stem) in all numbers, gen/dative singular and plural e.g. ἁλῶν and ἁλσί(ν). But that is not true of the irregular nouns like πατὴρ e.g. πατράσι(ν), not πατρασί(ν). Any idea why?