The first thing to remember Jon was that this is not a true account of one mans life. It is a 'put-it-all-together-and-see-what-comes-out' type of song - which is often the case in shanties with a storyline.
Folks of the time (18??) would not have been unfamiliar with tales of coloureds living among native indians.
He was put on board of a 'whaling ship' because 'timber drogher' 'coal barge' 'three skys'l yarder' and 'bluenose schooner' do not fit the rhyme or the meter.
Mirimichi is where his ship happened to be at some time during the story - not where John came from.
Indian slave - why so unlikely? Many slaves were taken in by indian tribes and were retaken by white slavers.
Starved day & night? Merely an indication of bad treatment. To say that he was fed slops and meat that had past its sell-by date and then subjected to taunts and the occasional clip round the ear is hardly the stuff of old norse sagas.
Gatepost rhymes with ghost - its a natural, no?
Sitting on the hatch/main truck, wet & green. Its the stuff of nightmares. The writer is drawing us a ghastly picture to chill our bones.
The verses are thrown together, but in a narative order. Some verses may be added/left out and still the shanty tells the same story, albeit with a different story teller.
Examination of this style of shanty can be taken too far - it is not intended to be taken literally. For a prime example of a song which is clearly based on total rubbish, yet has been passed into common acceptance you need look no further than 'The Lion Sleeps Tonight' where it is stated that the lion sleeps in the jungle, the mighty jungle. Lions do not live in the jungle - they are to be found on the plains.
Wimoweh to you all :-)