Appear to be? On what grounds? Hell, we've offered him Substance of Shadow before when we first attscked clan mors with him. He declined because it'd interfere with his tricks.
Why would he ever spend his own favor on ulgu stealth items? Are you going go order him to?
Substance of Shadow is certainly an internal spell, as it changes the entire substance of the target with Ulgu. Doppleganger and the like don't seem to alter the target, just cover them with an effect from the outside. It's something only
@BoneyM can answer, for certain, but it's certainly plausibel that they're not all internal effects that would, for example, conflict with the Seed of Regrowth.
And as to why he'd spend his favour on stealth items, it's because there are limits to his ability to loot the skaven blind with his natural talents, and Mathilde should be setting a good example about the use and abuse of both magic items and the Lore of Shadows.
I would cite his competence. His skill and plans were never in doubt. You mentioned increasing his diplomacy when he doesn't need that help. He's already a better diplomat.
You question his skill, but he's already had his own plan and clearly investigates technology and uses bombs/incendiaries as a matter of course.
He's our equal. Treat him as such.
He's Mathilde's equal in rank, but I doubt he knows any battle magic, or owns any items of battle magic, or items of nearly legendary runic artifice. I actually think we haven't seen him with any equipment at all. Magical or otherwise. For all we know, his gas bombs are his Canadian girlfriend, something to convince his Gold Magister friends that he really is into manufactured objects, just like them. (And no, I'm not completely serious about that, but it is amusing that we have a Gold Magister here that is flexing so far from the stereotype - which is partially why I want to encourage it, as in her own way Mathilde is doing the same)
Also, he may be a better diplomat than Mathilde. I've managed people who were better at lots of things than me. That hasn't stopped me encouraging them to get better at those things, or arranging for training to help them do so. Generally you want experts working for you to develop their expertise, not stagnate.
I can't agree more. I thought that this ridiculous hubris trend got kicked down a bit when Mathilde got nearly done in by a dozen or two of your regular orc boyz.
I was clearly wrong.
Mathilde's a good warrior, but she's not Tyrion or Louen Leoncouer.
And even if she was, it would still be foolhardy in the extreme to challenge a mountain filled with deadly monsters all by herself.
There are specific reasons why a mountain of trolls are almost uniquely vulnerable to Mathilde in a way nearly all other enemies she might face aren't. It's nothing to do with how good a warrior she is. That's almost irrelevant here. It's the combination of Substance of Shadow and her sword against enemies who live in the dark and are notoriously stupid. Now, she will want to scout the mountain first to find out if there's anything problematic before going in, but when/if Belegar pulls the trigger this is a chance of Mathilde to really show off.