How many actions can the rebel transport take?

I am confused cause it says something about 2 additional attacks and then I read somewhere it depends on fore and aft cards or something just confused. So pretty much total actions the transport can do.

Is reinforce the only action that uses energy?

The transport can take one action per round by default.

I don't know if you have ever seen the raider or the cr-90, but those huge ships are so big that they are represented by 2 different ship cards that combine into one ship, and those ships can take 2 actions, one for each card. That is what that bit is talking about.

It's confusing because in the Huge Ship Rules, they talk about two sections and being able to take one action per section, and then reference actions for front and back cards.

Technically, all Huge ships have a fore and aft section, but not every Huge ship has a fore and aft ship card - such as the single card ships of the Gozanti and the GR75 Transport. The confusing line in the rules (currently on page 2 of the PDF) is "During the “Perform Action” step, ships with two sections may perform two actions." I think it should read as two ship cards and not two sections as the paragraph goes on to talk about how to manage the actions from the two ship cards.

You are right the text could be more clear, but the section header is pretty clear. Performing actions from a ship with two ship cards.

Is reinforce the only action that uses energy?

Reinforce doesn't use energy. You're probably thinking of Recover. Recover is the only action on a Huge ship's normal action bar that uses energy. However, some upgrade cards add other actions you can take, some of which involve spending energy. They always say so directly on the card, though.