Kashka does not join Grandma like a hero joins a unit. She is a passenger as for as the rules are concerned.
Look at it this way. If a Fireball is carrying a squad of Hell boy's as passengers. The Fireball can't use the Hell boy's hit points either.
As long as Kashka is embarked in Grandma, then Grandma can use Kashka's skills.
If Grandma is destroyed the model is removed and Kashka is placed on the square were Grandma was, as per the transport rules.
Read page 12 of Zverograd it covers all of this.
A unit may only embark or disembarked during a turn, they may not use any other actions.
The only advantage in having Kashka embarked on Grandma is that Grandma can use Kashka's Fighting Spirit.
For the record I don't see the benefit either! The cost of the models is 54 points!
I would put those points towards a nice chopper.