You're putting too much time into the classifications of things. That said, from my understanding:
An MC has a character arc in a given novel. That arc is how he changes. That given novel has a storyline (or plotline, if you like). That particular storyline, to that particular novel, doesn't change. It is the "story". It is what it is. It is "the male spy trying to save the world", from beginning of novel to the end, or it is "the young woman fighting off desires for an older man", from beginning of novel to the end. Each of these two novels may have little detours, sidetracks, but they come back to what they truly are, the storyline doesn't change. So it won't start as the male spy saving the world, and end as the woman fighting desires of the older man. Yes, these two storylines can obviously be combined, but the point is, the novel can't be exclusively about the first storyline for the first half or three-quarters, and then exclusively about the second storyline for the remainder of the novel.
Maybe it's splicing hairs, mincing words, damn semantics, but I feel it's the MC that has the arc in a given novel, not the storyline. To say that the storyline has an arc, I think you really mean the MC has the arc. To me, the story has an arc when it's from book to book, or episode to episode for TV and film; in this instance, that's where the storyline changes and has its own arc, over the course of several (or more) related works, the way an MC has an arc in one novel.
If you're talking about how the story has its peaks and valleys, how it "changes", then I think you're talking about the story structure (beginning, middle, end, rising and falling action, etc). Are you doing any great harm referring to a single novel's "story arc"? No. But it might confuse someone you're asking for help if you mix up the terms.
And, if I've mixed up the terms, I'm sure someone will be along shortly to slap... I mean, to kindly show me where I have gone wrong with my terminology.
UJ..? Where are you? What's your final answer? And don't paddle me too hard. That ruler in your hands looks like it'll leave a mark.
