Help!! Crazy boiler expansion tank setup.

Alright peoples, here is one I've never saw before. I'll attach the pic below for reference since I'm typing from my laptop. A new Laars mini therm boiler, very simple install.

The pump is 6 inches above the supply outlet which is standard. The domestic water feed is on the supply side directly under the flow check. The expansion tank is teed into the boiler bottom drain valve though. Is there any way this actually works? It's in between the supply and return side if you look at the technical drawing.

My dilemma: I don't want the expansion tank teed into the water supply under the flow check valve because it would be after the pump that is directly off the supply side boiler feed. Space is limited on the return side where i want the expansion tank. Is it alright to leave it as is temporarily? I would like to repipe the return side completely to make it right in the future with an air eliminator added. In the meantime will the loop be alright with the point of no pressure change at the bottom of the boiler? The home inspector also said the water heater can't be piped into the boiler flue since is has an automatic damper. Never heard of that before but I'll listen to any advice you guys have. Natural gas natural draft for both. 6 inch flue 200k BTU combined.