Yogi's stealing picnic baskets again

Came across what is considered a grey area with rules.
1) The number one Nurgle spell requires line of sight. so can you cast it into combat?
Refer pg 107 of the rulebook.
This has not been FAQ'd, but other spells with same targeting restrictions have been and the GW ruling was that they could.
2) when applied to a character on a mount, is the result randomised? If the mount is a steg?