Yes, baby striders should be able to automatically dismount once they grow up.
I think this is intentional, but I could be wrong, especially since a player can force a villager out of bed.
Alright, based on where Wart Blocks generate in either nether forest biome, I believe that (like leaves) only wart block that was placed shouldn't decay, and wart blocks only won't decay if they're touching the following: Stem, Netherrack, Nylium
I included netherrack and nylium because some blocks of wart generate as part of the terrain. They would be touching netherrack/nylium no matter what. Plus, it was intentional for nether fungus to grow regardless of space, which is why some huge fungus grows into the ceiling of the nether, in which case any remaining wart will touch the netherrack and not the stem. Therefore, for it to decay it ought to stop touching netherrack. Get the idea?
Finally, since nether fungus comes from using bone meal on nylium, good ol' nether wart can come from decaying wart blocks, to finally confirm that crimson forests are where nether wart comes from, and is where nether fortresses get it from (lore wise?). As for warped wart blocks...we'll let Mojang decide that. For those of you expecting warped (blue) nether wart, I suggest thinking about Nether Sprouts, a warped forest root which, based on the very name, might be foreshadowing something. (It's called NETHER Sprouts, not WARPED Sprouts, that's why)
I don't think riding baby striders are supposed to shiver. I think they can be fine being on top of its kind as well as being on top of lava.
Personally I think Striders shouldn't shiver when on top of magma blocks.