What would happen if I roll a natural 20 while performing a shield slam or shield snag?
Would the 20 count for auto hit, critical threat, and the attack roll based Combat Maneuver?
Shield Slam's bullrush lets me push a creature an extra 5ft for every 5 I beat the CMD. How would the 20 play in this? Take 20 add attack bonuses vs CMD for bullrush?
Shield Snag
Any opponents hit by your shield bash are also targeted by a free disarm attempt, substituting your attack roll for the combat maneuver check. This disarm attempt does not provoke an attack of opportunity. You cannot use this ability during the same round as a bull rush attempt made with Shield Slam or with any other abilities that affect how your shield bash works.
When using a shield, you gain a +4 bonus on Strength checks to avoid being blown away by a strong wind and on Climb checks to catch yourself on a wall or slope when falling.
Shield Slam
Any opponents hit by your shield bash are also hit with a free bull rush attack, substituting your attack roll for the combat maneuver check (see Combat). This bull rush does not provoke an attack of opportunity. Opponents who cannot move back due to a wall or other surface are knocked prone after moving the maximum possible distance. You may choose to move with your target if you are able to take a 5-foot step or to spend an action to move this turn.
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If you roll a natural 20 on the die with the shield, you will use the same roll for the shield slam or shield snag, so it would be an automatic hit. There is no critical hit for a bull rush or disarm, that's as far as it goes...but even if there was an ability or feat that that used a crit roll, those would be rolled separately unless the feat specifically called them out.
You are correct that the die roll being a 20 does not give any additional benefit as far as calculating the 'push' distance for the bull rush. Take 20 and add any applicable modifiers.