I'd say our odds of getting Halla to 40 are pretty decent, honestly. It's her odds of ever hitting 50 that are not great.
I'm not as sure about that as you are. Ignoring the fact that we regularly throw ourselves into peril against people as strong or stronger than us, I imagine each stage of cultivation has more traps than the ones before it. Halla may have been able to dance amongst the landmines so far, but that luck will run out one day and Ymir will squash us flat.
Planning this far in advance seems kinda premature. Like, what happens if Halla or Sigurdr completes Jarlsoul and builds a power base? It's by no means impossible, and would derail this plan a fair bit. We can plan one life out because we know the kid in question's Traits, but planning two is basically just pure guesswork.
I don't really see it as premature. Having long term plans in this sort of generational quest seems like a good idea. You guys planned what you wanted for a successor long before Sigurdr came along and fit the mold you had cast for him. Alectai first came up with the idea of a wandering cultivator for life two before Halla even got married.
I also don't see Jarlsoul being completed in Halla's life as being particularly likely? For one thing, we're having enough trouble fitting all these damn characters on screen as it is, and we want to stuff in another 7? I think IF would explode.
But also, even if Halla does manage it, it won't help because our most likely chosen successor has already been born and therefore can't get the trait from her. We'd have to start all over again, and a wanderer who doesn't put down many roots isn't really the sort people gravitate towards as a leader. Even spending time amongst the Varangians's isn't a guarantee of it, since none of the former guards we know have it (I imagine being sworn to the Roman Emperor changes how the system works).
A third life, one who we can set up from the outset to fill the role would likely be our best bet to truly seize power from somewhere. If nothing else, we can make them slightly less personable so they don't need to know each and every single person who serves them's deepest and darkest thoughts and fear, thereby saving poor
@Imperial Fister some work.
If you'd like a little bit more inspiration, Iceland will soon be discovered in just a few short decades (in real life, Iceland would already have been discovered and well-colonized by now, but in NorseQuest I'm setting it at around 900 AD (9000 AD in-setting)). If you're at all familiar with Viking Age Iceland, you'll know that it functioned a fair bit differently then Scandinavia did.
Iceland could be a really cool choice to set up shop in, but it does come with some downsides. The upside would be that it's far away, basically unclaimed, the perfect place to build a power base and start changing how our people view the world. Hell, being far away from traditional Norse lands might even help dampen The Enemy's like being in Christian lands does.
Of course, the downsides are basically exactly the same. It's far away, undeveloped, and whatever changes we make there will have very little impact on wider Norse society unless we force them to accept them somehow.