Hi, I'm new to this quest, so please take it easy on me
That said, the most relevant part of the question here as I read it seems to be why Ranald specifically, so how do we answer it without outright spilling that he's our best bro?
I'm thinking a combination of factors: The biggest issue with Grief, as it is written, is that we do not want the extent of the personal connection to be on file, so we add just a bit of piety to circumvent that. Not to mention that in the context of who Mathilde is talking to and who she worships it's practically her duty to throw shade at Sigmar.
[] A combination of factors.
Mathilde was still quite new to knighthood yet fought to defend her liege until reinforcements arrived, only to watch him slowly die. A combination of grief and inexperience gave the impetus for change, her principles as part of the Gray Order gave it shape. She's loyal to the empire and it's people, so who does she complain to when Sigmar isn't? Given how Sigmar openly muscles out other gods but then abandons the most devout followers, Ranald the Protector seemed to be the answer.