panel tilt

This is a good observation, but what if you can't physically add a tilt to the side? Would it help to add an elbo on the hot water side so there's "upstream water" as well?

An elbow will not help. All piping (inside the collector as well) in a hot water installation must always be installed in such a way that it inclines in the direction of water flow, otherwise you will get air trapped inside. On all the highest points in the installation you should have air release valves to relieve air build-up. If you mount the solar collector flat against a pitched roof, just mount it in such a way that it is not square/parallel to the roof top and bottom and to have the hot water side higher than the cold water side.