RE: Positional apneas suddenly worse post flu
The Caldera Releaf Collar is great, but they do not go as high as some other brands. Measure by the chart you posted to see how high you need, and see if the Caldera is that high. If not, you probably need another brand. I too found the Velpeau not as comfortable.
I'm a back sleeper due to pain and didn't like wearing a collar, so I bought a child-size pillow (they are always flatter) that has a raised front edge. It holds my head tipped back slightly and has solved my positional apnea. My pillow is no longer made, but Amazon has one that looks like it might work. Here's a link in case you are interested:
https://www.amazon.com/MLILY-Adjustable-...r=8-6&th=1
RE: Positional apneas suddenly worse post flu
Also be mindful of that even though you are over the Flu and feel better. You can still have some lingering phlegm roaming around.
I have the Caldera. But found that while it worked pretty well. The foam was weak enough that a few times I had managed to over power it and still went fetal and had positional apnea's. Another member here showed me "the eliminator sleep cushion" Very comfortable as the foam is only out front and just has a strap out back. Def cooler sleeping. One caveat is that the foam is very firm and if like me. You roll onto your belly. You can choke yourself into apnea's.
Eliminator sleep cushion