Signum vs. Cinque. I mean, what about igniting the sword and frying Cinque's hand in the process? Or what about skin-tight powerful shielding spells like Panzergeist?
The flaming sword wouldn't have mattered in the end, as Cinque was shown able to regenerate her own hand from the explosion. As for Panzergeist, it needs Laevantein to pop a cartridge to power it up. Having a bomb that's going off in your hand pump magic through itself and into you is probably not a good idea.
Signum didn't know about Rumble Detonator at the time, raising the question about why she didn't ignite Laevatein; Shidden Issen is not a small spark, it's a spiral stream of flames that allows Signum slice through barriers and heavily armored foes. If anything it could been a good method to get Cinque's hand out of the way (even if the cyborg has a Healing Factor, the regeneration isn't instantaneous and the time she takes to regenerate could've allowed Signum to make distance and rethink her battle approach, not to mention the crippling pain Cinque experiences every time she regenerates. About Panzergeist, while it's true it seems to be cast with Laevatein's assistance, it doesn't need a cartridge at all (as seen in A's ep. 2).
My guess, Signum didn't want to cause severe injuries as she knew the last time a Chaos power user (Nanoha herself) got severely injured, really bad things tend to happen. And regarding Panzergeist, I doubt it would have made a difference anyway. This is Chaos magic, the power behind the spells like the Ragna Blade that can tear through dimensions.
That's assuming Cinque directly attacked her with Chaos Magic. Rumble Detonator just turns metal into explosive material, the same kind Nanoha and Co. has the ability to shield from for years(probably even tank missiles will fail to do effective damage to a well placed Panzerschild). Seeing as Panzergeist was able to shield Signum from much worse in the past i think the point remains. Still, i give some credit to Cinque as it was stated by canon that her longcoat have some AMF capabilities which can be handwaved as the reason of why Signum's defenses weakened at such short distance from the cyborg. Well, that and the fact that Signum's human body is a lot less tougher than her canon counterpart.
There's a popular guessing that theorizes Laevatein isn't really made of real metal, as Devices in reality are magical constructs made of mana shaped into a solid substance, which will make Rumble Detonator's effect unusable on those (and also could explain why Cinque just didn't blow up all the Devices she faced on the canon). But seeing as BTF works by its own set of rules, I wonder if Laevantine can be considered a legitimately real metal sword in this verse. It sucks for her at this moment, but if Laevatein is a true sword that could give an entire new dimension to her future fight against Cypha (should the writer decide to include that in his canon).
Signum was one of many in the TSAB with the Blaster Metal upgrade (which she was using in this fight anyway). Her Device, if it didn't have physical material in the blade, probably does now.
According to Chapter 50, Resurrection magic can't regenerate lost body parts, but canon openly says the Wolkenritter were able to be recreated good as new even if their flesh and bones are torn appart (a past Arc of BTF has Vita doing exactly this after her face was basically ripped apart by some guys who captured her). I think it would work well if the reason behind Signum's injuries not regenerating is alluded more to the fact that she's now human (the Wolks become human in BTF-verse) and thus, resurrection magic is inaccessible to her.
I think you mean that Resurrection magic works just as well for her as it does for any human.
Well, yeah, that more or less comprissed what i meant to say. Being a human now is what prevents her to use her old Wolkenritter's resurrection spell that restore her to mint condition.
Technically speaking, resurrection magic can regenerate body parts, but only if anything remained of them. So an arm that is barely there could be regenerated given sufficient power, but an arm that was completely blown away can't. It's as if the body was rebuilt to match the blueprint that is inscribed on the original body, with parts of the blueprint being lost when any body part is lost completely.
The Wolkenritter entire bodies had been completely destroyed in the past several times alongside the destruction and death of various worlds while their resurrection spell has always been able to return them as good as new no matter the degree of destruction(from a certain POV it's almost as if the Book had a reliable memory of the constitution of their complete bodies). Some people(and partly this fic) implied the Wolkernritter fought and sometimes lost during various wars where is somewhat safe to assume they lost body parts(specially Vita and Signum being front fighters and all), yet they always had been restored completely. So yeah, the more plausible explanation for Signum being currently unable to fully recover her body is because they don't have access to such powerfull magic anymore now that they were downgraded to mere humans.