And you didn't even give a reason that plastic would kick us out of the flight envelope if we ignore those flaws in the argument.
So your very last line is true but you have done basically nothing to demonstrate that we have to choose between the two.
So instead we should look at actual harm and/or cleanup cost.
Looking at what it would cost to clean up doesn't have to mean actually cleaning. But let's remove that part, because the important part is to look at actual harm.
Especially because if you can't tell me the relationship between plastic levels and harm, and you can't tell me how much is too much, what if I decide we've already broken the invariant and there's no point in reducing any more?
I only suggested looking at cleanup cost to get a better understanding of how to handle the harm. Not because we have a magic list of which things must be cleaned up and which things don't need to be.
Because you'd need such a magic list. Otherwise how do we know that we don't need to blow up all our houses because they're violating an invariant, or something similarly crippling? What if Earth can only safely supply 10 million humans in the long term, for a fun possible invariant.
You wanted a definition of externality, I gave it to you. There's a function that gives you the upper bound on the cost of an externality, which for lack of a better term, we can call the clean up function. This is the cost of returning the system to its original state.
You don't need a magic list, you need a way of demonstrating that certain actions are within the flight envelope of the environment, ie, there is some corrective force within the environment that maintains the invariant for you, and a policy of denying all actions that do not come with such a demonstration. The list can expand as scientific understanding expands, and we are allowed some leeway by the intrinsic stability of the system. If evolution could not absorb some unexpected perturbations, we wouldn't exist in the first place, but we should treat this as a finite resource, for which we do not have an amount.
Evidence that we don't do this at all is just you being in denial about the terribleness of the human strategy.
Imagine, for metaphor, that you are in some (solar powered) alien plane, and the pilot dies. You go into the cockpit, knowing that you just have to keep the plane flying. You make a mental note that all of the instruments on the panel are fixed, and the position of all of the controls. You decide that you want to fly faster so you fiddle around with the contrls until you can tell that the landscape is going underneath you faster than it was before. You note that some instrument on the panel is increasing. Do you A) ignore it, or B) return the controls to their original state?
Your example is disingenuous because a cockpit instrument by definition monitors something you want to pay attention to. We can measure and record many things, most of which are probably irrelevant to many problems and their solutions.
Ultimately, all utility functions (strategy space to R), when integrated over the rest of time, produce either a finite value, or an infinite value. Trying to figure out which, for a given strategy, is in general undecidable.
Numbers sort of break down at this point. Or at least numbers as linear constructs. Humanity lives in a dynamical system, and as long as that dynamical system is stable enough (eigenvalues all near 1), we know from observation that evolution (in the biological sense) is capable of maintaining the invariants required to sustain human life and civilization. I will call this the flight envelope of life on earth.
This is an extraordinary gift. If I can prove that Humanity's strategy preserves these invariants, I can count on evolution to extend our presence more or less indefinitely, at least up to the point where the sun runs out of hydrogen or we are struck by some very large celestial object.
The squandering of this gift, by failing to maintain these invariants, has a literally incalculable cost. If we voluntarily exit the flight envelope of life, it is impossible to tell what will happen as a result, but it won't be as good as whatever we can get by staying in it. So my back of the napkin math says that the correct price to place on disposal of plastic into the ocean is "don't". There is no world in which the marginal utility of 50 trillion plastic bottles is greater than staying in the flight envelope.