Fantastic Letty.

I could chase her for hours before dying of hypothermia...

Translation please.

Mastine said:
she had to tell cirno winter was ending to go drink with Yuuka?

I read it as Cirno getting less and less demonstrative with her protests over Letty leaving when winter ends, over the years.

Kind of like when children grow up and don't really cry when their parents go away on a trip or something.