OP, I can confirm that your reading of the law was correct and that you are allowed entry into Schengen even after expiration of a visa, and that any time spent in the Schengen zone on a specially issued visa does not count against the 90-in-180 day total. (You found the relevant statute yourself, but there is also specific verbiage to this point on numerous embassy websites.) I know because I was in the exact same position three months ago and had to research the same question.

Having said that, in the future it can be easier just to stay in the Schengen area and explain the situation to border officials on your exit. The law is a little grey in this area (whether you can just "stay" or if you have to leave/come back), but as you found out it can be easier to deal with emigration than with uninformed airline employees.

How to move forward: definitely lodge a complaint directly with EasyJet in the manner described by David-A above. Also, ignore what njf63 said as almost everything in his/her response is wrong, as David pointed out point-by-point in his reply. I would suggest an immigration lawyer, but I'm not sure the cost will outweigh whatever you'd recover for reimbursing your out-of-pocket expenses. You have an almost ironclad case that your documents were in order and that you presented yourself for check-in in a timely manner, so denied boarding compensation certainly is in order.

Good luck!