A ” person subject to immigration control” is defined in S115 of the Immigration and Asylum Act 199 as a person who is not a national of an EEA state and :
1)requires leave to remain in the Uk but does not have it or
2) has leave to enter and remain in the UK whihc is subject to a conditon that he does not have recourse to public funds or
3) has leave to enter and remain in the UK given as the result of a maintenance undertaking.
It looks as if this person is not subject to immigration control as there is no public funds restriction on the visa.
As for habitual residence, a period of two years snce the visa was granted should be sufficient to determine that she has been resident here for an appreciable time. Given that she has also been working, the circumstances add up to habitual residence here.
I take it that there are no other stamps on her passport showing long periods where she returend to the Ukraine.