Obviously some goes out laterally, but next to the ground is where the lowest flux of shrapnel will be.
You are misguided on point 2.
This link has a ton of information and photos.
pmc.ncbi.nlm.nih.gov/articles/PMC5514183/
Fragmentation grenades are designed to throw shrapnel in “all” directions. A majority of them will be up from the point of the floor however there are plenty of fragments that will still be thrown laterally across the floor.
That being said you would have a higher chance of survivability laying on the floor.
deranger@sh.itjust.works 2 days ago
fonix232@fedia.io 2 days ago
Hitting the deck is still useful. Standing, your entire height and width is exposed to the source of shrapnel. Laying down, especially feet towards the grenade, reduces this surface greatly.