There's no discussion about ECB priorities. Whenever there's a conflict between inflation and growth, it is obliged to opt for the inflation control target 2% p.a.. Period.

"Thus, consumption/investments cannot increase since the loaned funds are already "investments". It is a zero-sum game as you've presented it."

You confuse world and non-U.S. world here.
When the non-U.S. world reduces its lending of money to the USA, the non-U.S. world would have more money left for consumption/investment. Simple fact.
That's what you denied (instead, you claimed they'd have less money to spend). I did never write about increased world-wide demand here.

And the present situation of assumed good ROI in the U.S. is a present situation, one that would not persist in the scenario we were talking about; an economic crisis in the U.S. that shrinks its huge imports.