I’m pretty new to magic and whenever I think I know what’s going on I usually don’t. Here is the situation thanks in advance!
Assuming a card like Jokulmorder comes into play and has the text: “comes into play tapped” and “Doesn’t untap during your untap step” and “whenever you play an island, you may untap” to paraphrase.
My question is can you use a creature ability to untap a card like that without meeting the conditions?
The answer is yes - abilities which specifically let you untap a target are ways to get around some of those restrictions. In particular:
"Comes into play tapped" means exactly what it says - when it comes into play, it is tapped. Nothing stops you from targeting it with an effect that untaps it.
"Does not untap during its controller's next untap step" puts into place an effect that gets checked at a specific point, i.e. in your next untap step, when you would normally untap it, you don't. So if you untap it with another effect, this does nothing - but if it then gets tapped again, it will still not untap next turn.
Having an effect that lets you untap it doesn't stop other effects also untapping it, you just have an additional way of doing it.
Absolutely you can! The first effect you reference on Jokulmorder is a replacement effect that modifies how the creature enters the battlefield - it makes sure the creature enters tapped. The second effect is a static ability that says it doesn't untap normally during your untap step. The third effect you reference is a triggered ability that defines an instance when you would get to untap it.
None of these restrict your ability to untap the creature through other effects like Seeker of Skybreak's activated ability.
I can't think of any creature that restricts untapping in the way you're concerned about - it would need an ability like "cannot be untapped" or a replacement effect like "if it would become untapped, instead it...", but it would make it quite the difficult creature to use.
