If you swore an oath to not raise a weapon against another to an ancestor spirit that is very big on pacifism, it still strikes me as rules-lawyering if you cast a MuCo(Te) spell to turn your nails into knives.
The spirit you swear your oath to is going to matter. This specific example is Clan Arelie, probably the most pacifist clan out there.
If this was something a player in my saga tried on, I'd have their oath-granted virtue (affinity with mentem) fail because they're not respecting the spirit of the oath. I'd then have an 'ancestor spirit' show up and offer them a secret ritual more appropriate to their demeanour that grants affinity with mentem.
Said subsequent spirit might well have 'false' and 'god' in its SG-only descriptor, but I wouldn't be telling the player that.
There are absolutely other spirits out there who would react differently. I wouldn't do this for every oath of this kind, but in this specific instance I would. If the character wants to 'game' for power, I'll introduce them to the experts in power-gaming in Mythic Europe and let them work out a mutually beneficial agreement for easy power.
I'd probably put that one in the 'sin' camp too. Churchmen and those operating under oaths of the Church are far from immune to sin, after all. See exhibit A: 1204.
personally I'd put a finer parse on it than that- claws from a heartbeast I would not count as weapons (or lycanthrope or shapeshifter...) but claws acquired from a spell I would because they are not a 'natural' part of that character.
Well the Clan Chief is listed as Ursula Densacer who has a wolf Heartbeast, so that (having a Heartbeast with claws in a pacifistic clan*) doesn't seem to be an issue.
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As to the weapon issue, my view is that the wording of the entry, "The initiate vows never to raise a weapon against another being...", seems to have been carefully chosen to allow the latitude of using weapons against "non-beings" (which is essential for some tool use), to allow the use of things like claws, teeth, talons, beaks, etc. in combat and to allow many forms of combat magic (certain spells are probably no-nos, like the Crystal Dart.) Even an empty hand can do damage, after all and the wording of the entry could have been more restrictive "The initiate vows never to commit an act of violence...", for example.
The consequences of breaking the oath are outlined later on in the chapter, under "The Oath of (Name)" heading in the "Initiating the Clan Mysteries" section, pg. 25.
Still, the presence of that particular initiation means there are degrees of pacafism within the Clan. The members with a stronger streak of pacifism would take that path, while others may select a different one emphasizing another aspect of the Clan (i.e. Clear Thinker or Piercing Gaze) more in line with the nature of their heartbeast.
Edit: That's just one possible way to interpret Clan initiations. Another would be that, as you progress within the Clan philosophy, you add additional initiations. So in this case the Clan Chief would indeed be an initiate of all the Clan mysteries. I haven't closely read those sections of HoH:MC recently, so I'm not sure which (if any) interpetation would be right.