Are action cards the only actions that players can take? Or can players take other actions as well within the scope of there skills, a more basic attack for instance?

In the deck of Action cards are a bunch of cards labeled "basic". These are cards everyone has access to, and don't cost an Action advancement.

There's a bunch of different basic actions, including one called "Basic Melee Attack". There's also one called "Perform a Stunt" that covers a wide variety of different skill uses. There's also a couple interesting defensive actions in the mix - more helpful standard actions for non-combat characters than most games will give you. You should definitely take a look through the basic actions if you haven't yet. There should be 3 copies of each in your core set, and 1 of each in the Adventurer's Toolkit.

The rules allow for Using a Skill, and the GM is adjudicator for what is needed for this to take place, what circumstance, what dice, and what effect those dice have when the roll is done.

I guess you could argue that "weapon skill" could be used as a skill check, and adjudicate the results accordingly. I do not however advocate this, or think it is with the scope of the rules to do so.

I am fairly certain that what the rules intend you to do is indeed simply to use "Basic" action cards provided at character creation.

I think it's worth saying that the basic building block of dice-adjudicated activity in the game is the characteristic check. Pretty much anything* can be done using a characteristic check. You don't need the relevant skill or a particular action. Skills make you better, and actions give you a broader range of specified results.

*Anything that isn't precluded by a lack of expertise, like medicine or channelling.

It seems one could actually play the game without even using action cards then? (although I think I would keep them, I just like the idea of giving players the freedom to operate outside the action cards if they have good ideas)

Players are not restricted in using their action cards.

They have one maneuver and on action each activation, with the option of buying additional maneuvers while suffering fatigue.

Using a skill is one maneuver option. (e.g. climbing a tree, defusing a bomb)

And remember that the Perform a Stunt Action Card is also broad in scope, for doing creative things with your character's abilities.