On Arena packs there are only 2 uncommons, and there are 20 uncommon planeswalkers from 80 uncommon cards in War - and those 20 would be guaranteed at the expense of other uncommons (mostly).
So that way, you will be opening much more Dovins than their vetos
And as you've said, in the normal boosters there are guaranteed planeswalkers.
EDIT: I do not use detailed math, but:
Uncommon PWs: 20 of 80 uncommon total
Rare/Mythic PWs: 16 of 68 rare/mythic total
Chance that you haven't any PW in 8 card Arena pack: (80-20)/80 * (80-20)/80 * (68-16)/68 = ca 0,43
So, chance, that you open one or more PWs is 57%
In classical 15 card pack you have 100% guaranteed one PW, in Arena 8 card pack there is 57% chance.
I would think Nicol will be stripped of spark (he hasn't got the Gem of Becoming here, which may or may not be the same thing) and will be locked in Meditation Realm. So returning him to the story would require resparking.
But my pet theory is that only Ugin has a spark - Nicol wasn't a planeswalker originally, he just stole the spark from that "demonic leviathan", and kept it within the Gem of Becoming.
Ral, together with Niv-Mizzet tried to run the "secret project" - to grant Niv-Mizzet the power of the Guildpact for him to be able to deal with Bolas. However, Dovin managed to sabotage this, exactly as he had done last time.