Yes, I'm certain of the I because it comes from the across clue:
Pot fiasco is risibly awful by that very fact (4,5) and I have IPSO FACTO . That was an easy anagram. :-)
I think you could be right there. I'm guessing the "first-class" bit is the AI, going north = turn upside down. Not sure where the fish comes into it though.
I think I've just got 14a. Utter an endless bitter tirade (6)
I've put ARRANT
That would fit with INDIAMAN as the 7th letter would be the 1st letter of ARRANT
I don't know what the connection to Pilate is (unless it's just being in conflict with the Romans) but the wordplay fits -
'similar' = SAME with 'can' = 'tin' (turned)= NIT inside.
The name Pontius Pilate provides two valuable clues to his background and ancestry. The family name, Pontius was that of a prominent clan among the Samnites, hill cousins of the Latin Romans.