I think you could make that complex loop work, but you would need it to be removable so you can physically tip and shake the radiator with the barbs pointing upward. Having the ability to tip and shake the reservoir would also help getting it filled, but it sounds like the bubble is stuck at the radiator plenum.
It would be helpful to see actual pictures of your problem..
Plenum chambers have a large x-sectional area which lowers velocities at that point.
24v will not help the 655 where it's likely operating for you, only the strong model really benefits from higher volts. I've only found very minor gains at very low restriction (past 1.5gpm)with the vario using 24v. You've got a lot of restriction and likely hovering in the 1 GPM area, so extra volts will do nothing worthwhile.
Second pump in series would probabaly help, but lots of bleeding guides I've read have the same recommendation about bleeding the parts outside of the case to prevent issues like this. Second pump would net around a 30% flow and velocity gain, but it's not guaranteed to fix the setup problem.
If the bubble is stuck in one spot and the pump is silent without any air caught in there, this is a combination of rad setup and lower flow rate problem.
You can either work on the setup to make it removable for bleeding, be willing to toss the case around as needed, or "try" a second pump in series and hope flow is high enough to flush the air.
I would lean toward making it removable...that's just general good bleeding practice and should solve the inverted rad plenum problem you have.
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