I have two different issues:

1) When I cast my spell, even though I have set a button for both On and Off state, the button disappears until the Behaviour isn't applied anymore.
2) I'd like one of my abilities to be castable only a certain amount of times (and it should recharges based on power-ups the players would collect).

Did you set up the command card correctly? You need to actually place both buttons on the command card.

For the second one look into charges. If you have played the secret campaign mission it is what is used there.

Basically abilities will have a charge cost, you set charges through triggers (there is an action to remove charges, simply use a negative number).

How do I do so? I have placed the button itself (there's only one button created) in the Command Card. And on the Ability itself, under "Ability - Commands +", I have this button linked to On and Off states.
This means I have nothing to change on the ability that is already designed? Everything I'll have to do related to those charges will be done through Trigger Editor, right?

You need two buttons. They can be the same graphic and all that, but there needs to be two buttons. Set the "Ability Command" of one to the activate and the "Ability Command" of the other to the deactivate.

You will need to set the cost through the ability (using the Cost property) and then giving more charges is done through triggers.

Worked, thanks!

I did this, but it doesn't seem to work. Do you have the exact terms for the states and the exact location please?

Make sure the toggle flag is set on the ability

You mean, under the ability's field "Stats - Flag"? I just tried and it didn't work.

So you have both the on and off buttons on the command card and the buttons are set for the ability?

Wait... how am I supposed to set properly the on and off buttons on the unit's command card?

You put them in the same button slot, first on then off.

Fuck that, it won't work.

(I removed my buttons copies because I felt like they were useless)

Attachments:

Turn the toggle flag on, in the command card where you would have an ability (absorb) have two buttons (in the same square), one for the on and one for the off.

I guess it's this part I am missing...
How to set them so they correspond to on/off state? Do I absolutely require two buttons for a single ability or can I copy the button in the command card and change some stuff?

When you add a button, you select the ability, optionally select the requirements and then select the thing its going to execute. There will be two options in the last drop down named after the buttons in the ability which you set for on and off.

Fixed:

Use "Effect - Instant" and an "Apply Behavior" Effect instead of just a "Behavior" type Ability.

Okay, I'm at designing that famous power-up giving one charge to a spell.
I found the "Add charge for player" action... weird how this action actually removes instead of adding a charge! (Which is why you told me to set -1, right?)
Anyways, I do not know how to call my Seeker Missile ability with it... it asks me to enter text instead of selecting an ability.

The text comes from when you set the charge cost in the ability. There is a link property under the charge section, you can add your own here by clicked add, and that is the text you need to use.

is my action.
I did set "Seeker" as the text in the ability's cost.

Why isn't it working?