Can you extend the duration of the “Defend” action

To answer your questions, in order:

• Treats are common and cheap, so use them up without worry. Gems, on the other hand (these teach spells to familiars) are much more rare. Be wary of how you use these.
• You can't undo treats, but don't worry about it. You'll be able to tame another version of the critter if you don't like how you allocated points. Don't worry about keeping your starter familiars either, you'll find consistent upgrades along the way.
• As I mentioned above, gems are what teach abilities, not treats. It will be clear to you.

You can use the "All Out Defense" command (this is a story reward) this makes your AI party members focus on using the Defend/Evade commands and will also focus on healing, but it really burns through MP.

Like other commands there's a timer, so you'll have to select it again if you need it, when it run out.