Is a person held accountable for giving in to disbelief-inducing whispers, and what are the implications of then believing oneself to be a disbeliever and committing forbidden acts? Is it advisable to change one's residence due to having committed major sins before and after becoming religiously observant?

You are not held accountable for intrusive thoughts as long as you dislike them, but you should repel them and strive to get rid of them, because succumbing to them leads to great evil. You are on the right path as long as you strive against these intrusive thoughts. Beware of abandoning the struggle against them lest they intensify. You are not required to leave the dwelling where you committed sins, and what you said or wrote under the influence of intrusive thoughts does not harm you.