I could put forth a guess or two on attempts at action, but I am not familiar enough with the science to argue them wholeheartedly. But I can say the political decisions are rarely science based and more often based on perceived righteousness (which is why it's a tax and not illegal).
Color me unconvinced.
That's an assumption of bad faith. Now that is something that makes rational discussion impossible. You seem to see yourself as totally above the person talking with you, because you're just "rational". But you continually appeal to emotion yourself, and that emotion is fear:
"it's a slippery slope", "prohibitionists", "using the long arm of the law" (to describe a 5p charge), "regressive", "dangerous question","interminable lists of laws" etc
This is probably not related to plastic bags, but still interesting:
73% of deep sea fish ingested plastic