We still have to demonstrate that the choice to go cotton to avoid micro plastic release outweighs the tremendous water use that goes with cotton agriculture - water use that occurs in some of the most water scarce areas of the world.
And unless we're going organic consider the impacts of eutrophication and other chemical discharge/byproducts.
There isn't a silver bullet in material sustainability besides non-consumption. And we usually don't know (or agree) enough to weigh externalities against each other.
Its important that we study, assess, and (hopefully) mitigate the release/impact of micro plastics because their isn't a plausible path toward the textile product's industry to reduce it's carbon footprint in line with the UNFCCC without relying on recycled polyester.
I think at this point they've moved to just "intercepting" plastic before it makes its way into the ocean. From the parley website:
"In remote areas, we establish systems to intercept plastic waste before it ends up in landfills, gets burned, buried or tossed into rivers or oceans."
So still might be more interesting than the recycled water bottles everyone else is sourcing but maybe disingenuous for them to be using the "ocean plastic" language still.
Re: your final paragraph it seems like it could be a0 > A & b0 > B & ... if you're also trying to (or forced by the market to) minimize the sum of a0 + b0 ...