I've never worked with Kveik, so I'm guessing. Around 70F she should propagate, probably just not as fast as at 80-90F.
Are you using a stir plate?
You could wrap a heating belt, fermwrap, heating blanket, etc. around the flask to keep it warmer. Or put inside your ferm chamber, a fridge, or cabinet that's heated.
From Bootleg Biology: Original blend of yeast isolated from Norwegian kveik
Since it's a
blend, it's very likely by making a starter, or reusing part of a yeast cake from a previous batch, the balance of the strains in the blend will change, thus flavor and fermentation profiles will change too. There is nothing you can do to prevent that, or not in an easy way. Just live with it, part of the process, or use to your advantage.
Temperature may accentuate or subdue the growth of different strains in the blend at different rates too.