Thanks for an interesting topic.
From my reading of this thread, I think we are all a bit confused about the set-up. I think most of us consider a siphon to be a device which uses atmospheric pressure to push water up and over the tank edge, before discharging to the sump. The term siphon as you (and others) use it in relation to Herbie design, is really a fully flooded overflow to eliminate noise and micro bubbles. The only way that can work is if you have a high level overflow in conjunction with a control valve on the low level overflow (siphon) so that the low level overflow allows almost, but not quite as much flow as the output of your sump pump.
I don't think this can work safely without a weir. If you have a low level stand pipe in the tank proper, the water will drain in a power outage and unless you have a massively over sized sump, this will flood the floor.
I think there are three obvious ways to fix your problem;
1) Install a weir with Herbie overflows incorporating a control valve on the lower overflow,
2) Install more overflows to cope with the output of you sump pump, or
3) Install an NO (normally open) solenoid controlled valve in line with the lower level overflow, so that air is excluded from that line in the case of a power failure. In this case excluding air is different to preserving a siphon, but I can see where people get confused.
BTW I don't understand what you meant when you said that you had bulkheads drilled in the DT.
I'd like to see a photo too, because I'm not sure that I understand properly either.