I hate to disagree with a fellow United Methodist Elder. But I have to say that I'm not sure a couple of your points jive with the doctrine of our Church as in the Book of Discipline.
You seem to suggest that United Methodists believe that conversion has to be instantaneous. Am I reading you correctly?
In Paragraph 102 Book of Discipline 2012 under the heading "Justification and Assurance" it says, "This process of justification and new birth is often referred to as conversion. Such change may be sudden and dramatic, or gradual and cumulative."
As to apostasy, that is someone walking away from their faith. No we don't believe you can "lose" your salvation like a quarter lost out of your pocket. But United Methodists does teach free will and the ability to walk away from your faith if you so choose.
Read paragraph 104 Book of Discipline 2012 under the title "Justification and Regeneration."
"We believe, although we have experienced regeneration, it is possible to depart from grace and fall into sin; and we may even then, by the grace of God, be renewed in righteousness.
In the overall 'big picture' of what God does - is saved, being saved, going to be saved - it is a process. Conversion being a changed behavior. That has to happen over time in a process.
In the context of (becoming a Christian), the Biblical fact that 'justification' is often an instantaneous action by God - Yes! However, as we grow in Christ, that is also can be called conversion. For growing in Christ, yes, it is a process.
However, for that (becoming a Christian), that time of assurance - assurance can be instantaneous. If assurance is not instantaneous - that means we have to Earn our acceptance with God. So in the context of becoming a Christian for assurance conversion is instantaneous.
I would say Yes and No - Yes it is instantaneous in the context of (becoming a Christian) and for assurance. No, it is not instantaneous in the context of discipleship, growing in Christ (e.g., Matt. chs. 5-7), growing in our Christ-like behavior. I think I've answered your question, but did I answer your question.
PS: My early comments dealt with becoming a Christian. I was not dealing with the 'big picture' what God does in our 'whole' life sort of thing. I came from a huge works based and cultic group that even told me: "You cannot be sure in this lifetime where you would go to heaven." I found it disturbing, but it was the Methodist that helped me come into the grip of God's grace.
To God Be the Glory!