I think the best translation would be a literal translation: "Onko kello pysähtynyt"
Maybe they don't want to scare us with pysähtynyt just yet ;)
Onko kello seisahtanut? should be accepted.
Why does the "stop" translate to "seisoo" (stand) instead of "pysähtyä" (stop) which is more correct translate for the clock stopping.
Yes, that is not at all the most common way of saying this. I find "seisoo" as a little bit old fashioned way to say this. "Pysähtyä" on the other hand is always right and is used all the time.
I think they're just restricting all the sentences to the present tense at this time. I don't think it would hurt to introuce pysähtänyt at this point though, because we can basically think of it as an adjective.
Yes, finnish people would most likely say: Voi ei! Onko kello pysähtynyt?
Voi ei! Onko kello pysähtynyt? should be accepted.
Should it be translated to is broken/stopped rather than has stopped?