To add to the discussion of $27USD being the floor/ceiling, that is the max SP GM will pay for NewCo AFTER separation.

Following the Separation and the satisfaction of certain conditions, GM has agreed to subscribe for shares of Lithium Americas (NewCo) at the then market price on the date of subscription, subject to a cap of 130% of the Tranche 1 Subscription Price (adjusted for the Separation) in an amount equal to $330 million (“Tranche 2”).

Even though we, the shareholders, are still in the dark as to how the separation will be handled, I can't think of a situation where $27USD post-separation would be seen as a negative.