After the divergence, yes, they are. Before it, they are ONE timeline.
Yeah, I agree. I say they were pretty much the same timeline until the arrival of Nero's ship and its disruption to Kirk's birth. My personal theory is that the divergences gradually increased over time, but early on, the Prime and Kelvin Universes had many of the same events in common.
For instance, the big divergence we see early on is the destruction of of the
Kelvin and the deaths of several of its crewmen, including George Kirk and Captain Robau. Obviously that makes Jim Kirk's early life very different, with his mom remarrying when she likely didn't do that in the Prime timeline. So the Kirk we see in the Kelvinverse movies is a rather different character. Angrier & more rebellious than the "positively grim" "stack of books with legs" we were told about in TOS. We know from the recent movies that Kelvinverse Kirk joined Starfleet later and became Captain of the
Enterprise sooner. And Spock tells us that George Kirk lived in the Prime timeline long enough to see his son take command of the
Enterprise.
But it wouldn't just be Kirk's life who was affected. Maybe one of those crewmen on the
Kelvin who died was the person responsible for the Starfleet design aesthetic we saw in TOS. (We'll call him Lt. Matt Jefferies.

) If he's gone, the look and technology evolves in a different way, to what we saw in the Kelvinverse movies. Maybe Captain Robau's death inspired a friend or relative to join Starfleet when they didn't in the Prime timeline.
Roberto Orci stated that the Iowa Shipyards we see in ST09 were built in memorial to the
Kelvin. That means there's a whole bunch of Starfleet personnel assigned there that would be assigned somewhere else in the Prime timeline. Likewise with the Kelvin Memorial Archive seen in
Into Darkness. And that's to say nothing of the 800
Kelvin crewman that George Kirk saved. All of those people would be reassigned elsewhere, where in the Prime timeline, they would just continue serving on the
Kelvin. Maybe one of them felt their brush with death was enough to get them to resign from Starfleet.
But conservatively, let's say that the Kelvinverse has 1000-2000 Starfleet personnel doing something different than what they did in the Prime timeline. Those lives touch other lives, and
those lives touch other lives, and different outcomes occur. Couples get together that didn't get together in the other timeline, and vice versa. (We see this in
Into Darkness where Kelvinverse Kirk apparently had a thing with Christine Chapel, but didn't with Carol Marcus.)
So the two timelines grow further apart. I personally doubt that Prime Universe Scotty was ever assigned to that station on Delta Vega, for instance. Thus, he never met Kessner. Kelvinverse Pike wrote his thesis on the
Kelvin. Obviously, he wrote it about something else in the Prime timeline. Kelvinverse Pike may not have ever visited Talos IV, and definitely didn't get crippled rescuing cadets from Delta radiation on an old class-J Starship. There may not be a space station named
Yorktown in the Prime timeline. The USS
Farragut is still around in 2258, while in the Prime timeline it was destroyed in 2257.
But there were likely some things that still happened the same way in both timelines, even with the divergence of the
Narada. I have no problem believing the scenes of young Spock getting teased or of Spock refusing to join the Vulcan Science Academy happened the same way in both timelines. I don't see any huge contradictions there. The timeline differences just hadn't affected Vulcan in a big way yet. I personally think that the Prime Universe's McCoy probably also had his marriage fall apart in 2255. YMMV.
Yeah, there's some weird stuff, like the system of stardates being different, Chekov's age being off, or Sulu's sexuality being different, but overall I think most of the differences in the timelines can be explained by the divergences growing larger and larger with each passing year. The more time that passes, the further apart the two timelines grow.
In the end, Admiral Marcus is only irrelevant because we have no idea what his prime counterpart was like. We don't know what happened post-2233 to turn him into the lunatic we saw in STID, or whether Marcus-prime even lived long enough to meet his grandson. It's a blank slate, really. So in THAT sense, yes, Marcus is irrelevant, because for all we know, he died in the prime timeline before even knowing about David.
Yeah. But I personally don't think it's too big of a leap to say that Prime timeline Carol had a father who was in Starfleet, or even that she served in Starfleet for a few years herself.