It prob was lifespan cause they said the Ratson’s are cursed and since she couldn’t use her healing powers maybe healing powers are tied to the ability to turn into a human so maybe being a mouse for too long is bad? The snake clan took over her family in the og timeline and the only way they could’ve done that is if the entire family died so cursing them to live as mice maybe shortened their life span? Or maybe like irl animals she got so depressed over Sera’s living situation she eventually just died from a broken heart?
I would agree for the lifespan hypothesis, 10 or 15 years for a mouse are truly more than extraordinary (they live 2 years max). So maybe being unable to shapeshift does seriously affect their life expectancy.
Also as you mentioned, the girl only manifested self healing when she changed and her dad can not perform his function nor heal anybody while he is stuck, sot either thee curse has multiple effects, or the shift is needed to express divine powers ?
So did MC die in the first timeline because she was malnourished? It can’t be because of lifespan…