That if that was Sigmar's reasoning, then he's a bloody idiot. For multiple reasons.
First of all, because there was a very real possibility that Mathilde would turn to the dark arts as a result of losing Abel. Even if we assume that non-winning voters don't affect Mathilde's thought pattern (which I think they do), her mind was still heavily affected by the ambient Dhar in Sylvania. That's what "allowed" for that option in the first place. Let me remind you that "Imagine what Dhar could do in the hands of someone competent" was not a write in, that was BoneyM before he started taking suggestions.
Even besides that, right after you lose a loved one is also a prime time for the Ruinous Powers to strike. What if Tzeentch had promised her enough power to never fail another lord? What if Slaneesh had whispered to her in her beloved's voice? What if Khorne had payed any amount of attention when Mathilde was angry enough to threaten to murder any doctor she felt didn't do a good enough job? There's a reason why so many fall to Chaos, and it's not because they're secretly evil all along.
But let's assume that Sigmar knew that Mathilde would not fall to any temptations, that the attentions of any relevant Chaos Gods were far away and that none of the vampires and orcs that almost killed Mathilde since then wouldn't. Well, there's still the fact that Mathilde's little stunt with Mork's energy had a 1/6 chance of causing Ranald's death. While Ranald might not be the most reliable God, Gods who are willing to help humans for humanity's sake are still in short supply and really not something that should be risked on a 16.7% chance of failure! For that to in any way be a reasonable action from a non-ludomaniac, Sigmar would have to be able to predict the specifics of Ranald's Luck, which is something I would put beyond memetic Tzeentch. And if we assume he couldn't predict the luck of other deities, then there's also the part where Mathilde almost got turned into the prophet of Mork. And if there's one thing the Greenskin needed, it's their own Mathilde... Honestly, I actually like to read that, it sounds hilarious.
And even then, aren't we counting our journeymanlings before they graduate? Dwarves being saved is nowhere near certain at this point. Even in the likely case that we make it through the Waaagh without catastrophic losses or Mathilde turning with necromancy, we still needed to avoid the diplomatic clusterfuck that is Thorgrimm's everything. There's enough going on to justify Karak-8-Peaks getting their own army list seperate from the regular Dwarves, for goodness sake.
And once we've done that, we still need to actually help the dwarves move forward rather than just stopping their decline as we're currently doing. Which we're still not really any closer to doing.
But sure, let's assume that Sigmar could predict every single action Mathilde took, every single outcome of those actions and that we'll ultimately succeed in our unreached goals (which is a lot to assume). I would still call Sigmar a bloody idiot, because there's a very easy way to achieve all that without killing off Abel. First you heal Abel in a big, flashy way that makes your involvement not a question, and solve Abel's crisis of Faith. Then, when he manages a sweeping victory against Castle Drakenhoff, you give either Abel or Kasmir a vision that the Elector Count needs to send his shadow to help the dwarves around the time Belegar asks for help. Possibly be more specific if it's not forbidden by divine law. Boom, you have all the benefits of Mathilde going to help the dwarves on her own with the added benefit of an elector count that isn't afraid of mages and having you dwarf saviour not want to impale you at sight.
That being said, if BoneyM put out a vote asking if we would burn all of Mathilde's achievements in Karak-8-Peaks to the ground in return for getting Abelhelm van Hal back, I would vote for it in a heartbeat. Heck, I would do the same even if losing the Liber Mortis alongside any knowledge it has, would and could provide permanently was also part of the deal.
Also, if Sigmar's goals was to help save the dwarves, maybe he should've considered sending literally anyone to join us. Just a thought.
