Just a few comments without addressing specifically A and B in your OP but to dispel some of the errors I think I’ve read in some posts on this thread. Yes, a person can sin mortally or venially in thought, an interior act, without an exterior action. In fact, every mortal or venial sin that passes into an external action such as a word or with the members of the body is first mortally or venially sinful interiorly or in the intellect and will. For the will moves the members of the body and the intellect as well to their acts. There is no such thing as a true human action that doesn’t proceed from the intellect and will of the person. A sin is a voluntary action and this proceeds from the free will working with the intellect of the person. Since the will moves other powers of the soul to their acts and the body too, it is the first subject of sin or of a good action.
Thinking (an operation or act of the intellect) over some thought is an interior human act for we can’t think about something without willing to think about it. An act of the will is a human act too and the will does not will anything without something to will which something is provided by the intellect, namely, some thought or knowledge under the aspect of good. Again, all sins or good acts begin in the intellect and will for them to be called true human acts at all that are either praiseworthy or blameworthy. A thought that sort of spontaneously arises in the intellect doesn’t become either a good or bad interior act of the soul until the person realizes it and then proceeds to either to will to think about it or not. A person steeped in sin has kind of a disposition in themselves for such thoughts and thus they may be culpable to some degree for the very presence of such spontaneous thoughts. However, it takes an act of the will for any thought to be considered a sinful act or a meritorious or praiseworthy good thought.
Take, for example, a murder some person commits with the hand. Do we blame the hand or the man? The whole man is guilty for it is his soul that moves his body and principally his will which is a spiritual power of the soul. All sins are first in the interior of the person’s soul, namely, the intellect and will before they pass out exteriorly to the body or vocally in words. A person can pray to God interiorly without vocal words or using bodily members and this is meritorious, a good human act. Conversely and similarly, a person can entertain evil thoughts such as cursing God or planning to do evil and this is a bad or sinful human act. I think someone posted those words of Jesus where he talks about evil exterior acts proceeding from within a person. Some people can’t speak vocally or are paralyzed in the body but they are still capable of performing human acts with their mind and will such as making acts of faith, hope, and charity or the contrary of these and thus either meriting heaven or hell. Any person can make such acts or the contrary of these interiorly and in many cases to their dying breath and thus possibly save their soul or not. The angels who are pure spirits merited heaven or hell solely by an act of their intellect and will.