Combat Stamina is a separate ability; it allows you to allocate up to 5 stamina points to gain an equal competence bonus on an attack roll (including a combat maneuver check).
The Agile Maneuvers trick allows you to be treated as "effectively one size category larger when determining what you can target with a maneuver" for every two stamina you spend, equal to your dex modifier.
The two abilities require separate stamina costs; combat stamina increases your chance to successfully attack/maneuver something; agile maneuvers allows you to use the maneuver. As an example: I'm a medium creature trying to trip a Colossal creature. Normally by the trip rules I cannot do this, however the person with a +6 dex mod in your example can (6/2=3 size categories larger; medium ->large->huge->gargantuan -- one size category smaller than colossal thus legal to trip). I could then spend up to an additional 5 stamina using the Combat Stamina ability to help offset the tremendous penalty I face as a (still medium creature) trying to trip a colossal creature (which gets a +4 size modifier to CMD).
I would tend to agree on your first point however that the agile maneuvers trick should read "up to your dexterity bonus," since I don't really want to spend 5 stamina points if I have a dex bonus of +5, or if I'm trying to trip a huge creature and have a dex bonus of +6 or higher. Easy houserule to fix; I'm sure any GM would approve.
