P: HDR mask export for backlit prints

Ok, this is just an idea... I've just installed the Lightroom (I always used the LR Classic) and I was playing with the HDR feature... I have a lot of questions! (bad english spoiler)

1. What is the best export format? I tried the avif, but I can't see on my iPhone or with finder on my Mac. I can't share an hdr photo on instagram for example, the avif file doesn't upload well

2. This is the idea... Is it possible to export the "View HDR" mask as a "mask", alpha channel or something like that? I'm thinking on print it or cut a MDF to put light behind the print, what do you think?