Robert,
Not that you need any more, but here's a suggestion. Look at what you have in front of you and hook the thing up. Get the heat from the garn into the zones the quickest and easiest way, using the least amount of pipe and fittings and build a fire in your new garn. Run it for a while and then you can evaluate how it behaves. I wouldn't change a thing on your existing system, it works doesn't it? Leave the damned expansion tank where it is for now. At least you can get some heat while gathering hard data (not from a book or cyberspace) that is pertinent to YOUR system, not only theories. Agonizing over the exact positioning of every fitting and component can be frustrating, I found out and you are too, but it becomes easier to understand when you see it run. You will also find that it's not that big of a deal to not get it perfect the first time.
Mine is up and running and many things about the set-up are theoretically wrong but work just fine. I plan to change some things based on what I see happening, that will no doubt improve performance. But in the meantime, my needs are being met, my oil burner doesn't fire, ever, and that was the goal. I'll see what happens when it gets colder and plan adjustments/changes accordingly.
I think after getting varied and often conflicting advice from here and other places, there comes a time to make a move and find out for sure what's what. Everyone I talked with about my system, here and elsewhere, had advice and some were waay too focused on promoting snazzy controls, some wanted to talk at me about replacing part or all of my existing system. These guys wanted to sell me stuff, imho. Others wanted to talk about theories and books and were, as far as I'm concerned, trying to impress me with their great knowledge. Others were just showing off in trying to butt heads with other contributors here and/or their competitors out in the field and it got ridiculous.
Hook her up and find out for yourself!! It's snowing out