It is sin. I suppose the better question is, do "you believe in free will or determinism?"
We believe in free-will, so we define sin as "the actions of humans with free will who act in a manner that is opposite of the Will of God".
It only appears more coherent, because they re-define what works is. To them, any works of the faith is just some self-evident thing, and that if you were to fall or sin it simply means you were never saved in the place. Unfortunately, there's nothing in the Bible to support the "you were never saved in the first place" and plenty to support the "you were forgiven of all your sins and then chose to sin again".
The scriptures tell us that the pursuit of Heaven is like a foot-race, and to pursue our Salvation with fear and trembling, which is far, far from whatever it is Anderson teaches.
We differentiate between Mortal and Venial. Venial are sins that may fall under accidental, but may also fall under planned, it depends on the severity of the sin. Mortal Sin must always involve full knowledge of the sin committed, so it would -always- fall under "planned" in your understanding, and would involve a conscious rejection of what the Church understands to be a mortal sin.
Because Jesus is the first-born of creatures, before all creatures. The supposition, is that if Jesus chose to make His Mother immaculate, He could have, seeing as He was before Her. We also have it from Tradition that Mary was completely sinless in Her Life (St. Augustine, in particular).
Same thing, but minus the saved part. You are not to know if you are saved until Christ renders judgement, or absolves you of judgement when you die; we call this the "Particular Judgement".
For the Baptists, they read St. Paul's explanation of the justification of the elect; and we must conclude they are misreading him, by assuming St. Paul is claiming we can know if we are saved before Christ personally tells us.
In the same way, they are mislead in the reprobate doctrine, claiming that Christ can save all and forgive all sinners, except any sexual deviants by virtue (as it were) of their sexual deviancy. We say that even the worst sinners can turn to Christ if they completely repent, for Anderson, they are already damned.