[Nevadaref]

NFHS RULE
Rule 4, Section 44

ART. 5 . . .

A player holding the ball:

a. May not touch the floor with a knee or any other part of the body other than hand or foot.
b. After gaining control while on the floor and touching with other than hand or foot, may not attempt to get up or stand.

In your first situation, the player has clearly gained control while touching the floor with something other than a hand or foot, namely his knee. Therefore, he may not attempt to get up or stand.

In your second situation, going from one's stomach to one's knees is considered an attempt to get up. The player is only permitted to sit up after gaining control of the ball while lying on his back on the floor.