Smaller player models render incorrectly when sneaking/crawling

[muzikbike]

Sneaking results in being engulfed by the block below, whereas crawling results in levitation. These are effectively exaggerated versions of the vanilla bugs https://bugs.mojang.com/browse/MC-48191 (sneaking) and https://bugs.mojang.com/browse/MC-202579 (crawling; currently Won't Fix) - I'm not sure if either of these would be fixable without potentially causing compatibility issues with other player model affecting mods.

I also observed another thing that results from #1 - crawling puts the player model considerably behind where the player actually is (note the offset from the shadow):

[HowardZHY]

I have a question, is it compatible with Smart Moving?

[HowardZHY]

mostly works.
and fully compatible with real first person render.