Single Purpose:

Use at the start of your activation. You may use the same special action up to twice during this activation.

Grenadier:

(Special Action): Choose a space within 3 spaces and roll 1 red die. Each figure on or adjacent to the chosen space suffers (Damage) equal to the results.

I take this to mean a trooper can throw 2 grenades. Thoughts?

Yes

Since throwing the grenade is not an attack, yes...

No, "Grenade" is a command card. They are put into the discard pile after their use. They don't add the special action to your figure, but are a one time use.

RRG pg8, Command Cards Point 3:

• After the effect of a Command card has resolved, it is placed
faceup in the player’s discard pile.