This is like a giant game of Clue...
I'm fairly confident the pump is not the issue. I can open a port and water blasts out with as much force as I've ever seen. The boiler was also filled with softened water and then treated with corrosion inhibitor.
However, I like what you said about "the path of least resistance." It's leading me towards something...
With my T-stat broken (and now completely removed) the mixing valve is open. Ergo, it has 3 "out ports" and 1 "in" port. I'm thinking, perhaps, boiler water is going into the valve and then dead-heading itself again the top 2 "out" ports and then just looping down to the intended "out" port back to the OWB.
Here is a link to a Central Boiler brand mixing valve:
http://store.myheatingoptions.com/thermostatic-valve-kit/Look at the graphic of the valve cut in half, then imagine it without the regulating T-stat inside. Do you guys think I might be on to something here?
Would something as simple as a check valve fix this, or do I need to create a bypass loop to force water in the correct direction?