Service Valve Placement

I'm installing a Weil-McLain CGa-6 cast iron boiler. The piping system is a Monoflo from the 1950s. I will be using black pipe for the near-boiler piping. I plan to use Dan Holohan's pumping away setup but I want to move the service/shutoff valves to a different location and I'd like to get feedback whether the alterations would still work. Below, diagram #1 shows Dan's system as he presented it. I was attempting to change around my near-boiler piping to match this, but the original installers only placed an old school service valve (the kind with a square nob for a wrench) on one of the two supply loops. They also piped it such that it would be very difficult to install new service valves without taking apart a whole bunch of piping. I want to avoid that at all costs due to the age of the pipes and to avoid causing more and more problems. The system currently has two old school service valves on the return side of both loops before they merge into one and return to the boiler. It dawned on me that instead of trying to add service valves to the supply like in Dan's diagram, I could just install them on the return where the current old ones are (or beneath the old ones which I would leave in place and in the open position). This return setup is shown in diagram #2. Diagram #3 shows what I was originally going to do a while ago. That is, just put a service valve before the point were the supply loops diverge, and one service valve on the return after they merge. Basically treating them both as one large loop. But then I realized that during purging, the water would likely just go through the loop with the least resistance and not purge the other loop.

So, my main questions are, will #2 work? And am I correct about #3 that it would not work?