When you say that you cannot budge it........................?
View attachment 293981
Have you been levering from the horizontal arrow or the X?
If you treat the plate as sacrificial bearing in mind it is not for your Canon but a Sony..................at your risk, try inserting a broad flat bladed screwdriver in the gap indicated by the vertical arrow and lever at the same time as pushing against the place on the horizontal arrow.
Alteranatively, get a refund and and but one already supplied with a Universal Plate.
PS I have heard that the Manfrotto/AS Plates are poor on the AS usage 'compatibility' but never heard of one being so tight in use on the Manfrotto PL200 usage that you cannot disengage and release it
PPS I am at a loss to understand how in your post #82 that trying to fit the plate attached to your Canon has caused the jammed plate? Why, well your picture shows just the plate on the head with the grey locking lever 'over rotated'.
Such over rotation just(?) requires the careful and appropriate use of force to move it....... afteral you managed to apply enough to jam it, didn't you