One thing is certain:
You didn't succeed in pumping out the tank. However, if you were able to overflow the tank out the vent, I doubt that it was blocked--or if it was, the overflow was under enough pressure to break it open. One way to know for sure: SLOWLY & CAREFULLY loosen the deck ftting cap...you'll either experience an inrush of air (because the pumpout pulled a vacuum)...or--if the tank is pressurized, an outrushing of air...or even a geyser...so have the hose handy.

My best guess is that you only THOUGHT the y-valve in the pumpout line was in the "right" position. Check your y-valve to make sure which way the handle is supposed to point to open the discharge line to the deck pumpout fitting. "Conventail wisdom" says that flow should be in the same direction the handle points...but I've seen a lot of y-valves that were just the opposite.Other possibilities include a clog in the pumpout line...most likely that would be inside the tank, right at the fitting, but it could be in the line itself. Once you solve this problem, you have another one: if the joker valve in your head discharge were working the way it's supposed to, you would not have seen any backflow into the bowl...it obviously needs replacing. Since your toilet has prob'ly never been rebuilt, I wouldn't replace only the joker valve...I'd replace all the seals and gaskets in it.If you decide to modify the tank vent, I wouldn't put a tee in the vent line...I'd either install a second vent in the tank or re-route the existing line. And I would not put a loop in it because a loop prevents any exchange of air with the air/gasses inside the tank. Unless you sail with the vent thru-hull buried, you won't take enough water down it to notice.