Seoni has the power
For your combat check, you may discard a card to roll your Arcane die + 1d6 (+1)(+2) with the Attack, Fire, and Magic traits. This counts as playing a spell.
This means Seoni can discard any card to roll her Arcane skill (d12 + 2) + 1d6 for her combat check.
Of course, if she casts a spell which specifies she uses her Arcane die for her combat check, she would roll her d12 + 2 plus whatever dice are specified by the spell.
Right. So...
Situation 1
Seoni has cards in her hand which include Force Missile. She has to make a combat check, so she plays Force Missile which lets her roll 1d12 + 2d4 + 2. She will get to attempt to recharge Force Missile (and in Seoni's case automatically succeed).
Situation 2
Seioni has cards in her hand, but none of them would make a combat check. She has to make a combat check, so she uses her power and discards any of the cards to roll 1d12 + 1d6 + 2. She does not get to attempt to recharge the card she discarded for her power, even if it was a spell like Glibness.
Situation 3
Seoni has no cards in her hand. She has to make a combat check. She can't use her power, because it requires her to discard a card. So she must use her Strength or Melee. They are both 1d4 (Strength is on her card that way, she doesn't have Melee, so it is also 1d4). She she will roll 1d4.
I hope that helps. Good luck on your adventure.
I am honestly shocked at how botched some of these translations are. I don't think it's a case of "lost in translation" so much as it is just plain incompetence.
Translation can be really difficult if the translator doesn't understand the context really well. It's not enough to just know both languages.