Spent a little time on these pics...and here is my take. I can see both morphs(pied and bronze) of the Stewart Island Shag. I think I see a few Spotted Shags in the first pic, but I can not save pic and zoom in(I can only save as a webpage)
I believe it used to be Red-billed Gull on New Zealand, but at some point Red-billed was lumped in with and is now considered a subspecies of the Silver Gull.
Correct to leave as Gray-cheeked/Bicknell's. There are subtle differences that are very hard to distinguish in the field... even when in hand there is overlap in measurements and we have to leave as Gray-cheeked/Bicknell's... so, I question all IDs without a vocalization. Also 4 is definite Veery.
Yes most definitely a second year male.
In the fall of their HY, very few males show the start of some black spotting, but they are usually much more orange than the yellow of the females... but even that is not always the case..