Right, putting this one to bed.
mike, my work flow is functionally similar to
Either that or I download and edit on my laptop, airdrop (or android equivalent) across and then post from my phone.
I use Dropbox in place of airdrop.
Brian, instagraph seems to be aimed at Ubuntu phone users, a thing I wasn't aware existed.
However <drumroll>, I've sorted the main bone of contention, which was not being able to change aspect ratio when posting multiple images I've transferred to the phone by Dropbox. Turns out it was mostly user error and not realising the correct sequence of button pushes.
For anyone else (unlikely) to be stuck in the same position:
Select first photo
Change aspect ratio using the opposing arrows button
THEN click "Select multiple"
Select subsequent photos. These will all be at the same aspect ratio as the first. (I've yet to try it with landscape and portrait mixed in one post)
Post