Child alienation is when one parent belittles the other parent in the view of the child. Or when one parent denies access to the child by the other parent or minimizes access.
Giving a Get is supposed to be done, when a couple agreed to separate and have resolved all outstanding issues required to divorce and setup post-divorce arrangements. The Get is given as the last step following the resolution of all issues between the couple.
The halacha is that the Beis Din that orders a “get” will decide the custody issue, though obviously if the marriage is between two adults (referring to maturity rather than age), a Beis Din will just ratify the parents’ agreement.