OK in that vid do you have the clear hoses on both manifolds or just the one on the starboard side of that engine? If both have the clear hose do you see bubbles on both sides? Because bubbles in that hose can be an indication of exhaust gas getting in to the cooling water flow which usually means a blown or leaky head gasket. However, was the boat on land running on the water hose or in the water for that vid? Reason why I ask is because in the water if you see bubbles there, it points to the head gasket leaking exhaust gas into cooling water. On land this test can be inaccurate because when you run on the water muffs you can pull in a bit of air even when nothing is wrong. Any clear hose test is really only accurate with the boat in the water.
Beyond this, yours is a '98 correct? Volvo Penta used to use the same exact flappers that OMC used on their engines from '94-99. In '99 Volvo stopped using them because they can melt if you overheat or just the exhaust overheats and it can drop down and block the exhaust. When you pulled the elbow did you see a flapper at the top of the Y pipe? Was the little metal rod still there if there was no flapper? If so it could have dropped down and blocked the exhaust. Volvo put out a service bulletin that they felt these style of flappers were not effective at preventing water intrusion and should be removed. So that is something else to check.
Did you pull off the thermostat housing and check it for impeller bits? I would remove it test the thermostat (should open at 160) and make sure all the ports are open, they do start to neck down due to corrosion and any impeller bits if stuck in there will restrict flow. It would be ironic if all this trouble was caused by a bit of the impeller stuck right above the thermostat in front of the starboard side water nipple of the stat housing. This you would never see unless you pulled that rubber ring that holds the stat and removed it. Get a coat hanger wire and clean out the ports including the bypass in my diagram above. As far as pieces getting caught you are more likely to find them in the stat housing than the manifold, remember the flow from the impeller goes to the stat housing first, then to the manifolds (with a cold engine it flows through the little bypass port). If a piece of the old impeller got thru that, it could lodge right above the stat itself. Look at the diagram....
Ever try to run the engine briefly with both manifold hoses disconnected at the manifold and see if you get equal flow out of both?