Your symptoms are so close to the same ones I had, I am still betting that a flush of the coolant side would be in order. When I described my symptoms to a local Volvo mechanic, he immediately told me to a due a flush. So this must be reasonably common these engines. It worked like a charm. Your test on the removing the anti-siphon would have been more conclusive if you ran the engine at WOT for 45 minutes or an hour and it still did not heat up. Then I'd say you had some data.

Paul L