Birthright:

Min DC: 2 (+4 to DC roll, 2x Treasure Roll) [-30% to resolve]

Noob question: Do I undertstand this correctly in that this makes it an extremely difficult adventure to resolve? 4 extra DC means 2d6+4 compared to your EPL to determine success (avg effective DC 11 vs EPL) , and then with penalty of 30% at DC 2 will have a 0% chance to be resolved if you do succeed?

The first time, that is right. And the DC calculations are right. First DC2 attempt will return it to base 0

Correct, it is not a fun adventure from the looks of it.

0% on first attempt, second attempt will have a small chance, third attempt most likely succeeds.

So currently we would be looking at ~4 players devoting 3 actions each to have a reasonable chance at resolving something like this? Do adventures escalate the way non-adventure random events do if unresolved or will it sit there waiting for us to be appropriately levelled? Or will it disappear?

They escalate in their own way, the DC increases. Since there's no province connected to it, though, the worst that can happen if it's ignored indefinitely is that the problem could spread to inhabited provinces.

Why 4 players at 3 actions each? Could reasonably be accomplished by one player with three actions (providing they are suited towards the adventure and have a LT or two along for the ride).

Assuming generous level 6 with a +2 item and 2 NWP successes thats EPL 10. Assuming people's LT's are level 3, adding one lowers your avg party level by as much as it adds for the extra class and person doesnt it? Along with adding a 3 HD guy who can get gibbed by the hits that are rolled? Considering that as ideal solo attempt (+2 item and +2 from appropriate NWP) you need a 6 or less on 2d6 to succeed? Thats 41.7%. And if you did you would have 8 hits split between your 2 party members, likely killing your 3HD LT? This is not taking into account bad rolls either.

Wouldnt you need to recruit other PC's to make an attempt? What am I missing?

Because Lieuts will brig the APL of the party down by a similar amount that they raise it.

Level 6 Player
Level 3 Lieut

APL 5
2 Classes +1
2 People +1
NWP1 +1
NWP2 +1
Perfect Potion +2
Perfect Blood Ability +2

EPL 13 vs. 2d6 +4

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Okay it is done but still has a not insignificant chance of failure 10+ on 2D6, so 1 in 6 chance.

This requires the proper potion this early and a proper blood ability.

Without those you are looking at 2 players using actions so that their APL is ~6 to contribute to the EPL

Other things possible are magic weapons maybe two ECL 9 characters don't even need NWP.

Perhaps you also have Terrain Mastery - Permafrost NWP.
You could technically also actually have +4 from Blood at this point.

eg Detect Illusion + Animal Affinity -- or Detect Life + Animal Affinity

It is hard... on purpose... it isn't even technically required to be done. It's just eventually going to turn into an undead invasion, somewhere along the Permafrost border if not dealt with.