Is orthopnea (to the point you need to prop up on pillows in order to sleep at night) ever caused by acid reflux &/or severe, chronic stress & anxiety? to be clear: it's definitely orthopnea, not a panic attack, although i have plenty of those, too?

This could, indeed, be due to significant gastroesophageal reflux disease (GERD). You could prove this—and get relief, probably—by trying OTC Prilosec or other proton pump inhibitor (PPI) about an hour before bedtime. Should this not be effective, and it may take a couple of days, you should consider seeing you healthcare provider. Good luck for relief!