Okay, Let's see if I can address these
Is XI considered to be an alternate timeline within the original canon, or is it a separate canon?
According to the writers, and as hinted at in the movie, it is an Alternate Reality, created in 2233 with the arrival of the Narada.
If they’re the same canon, there are some contradictions that are difficult to resolve. For example:
The characters played by Zoe Saldana and Anton Yelchin bear little resemblance to the characters played by Nichelle Nichols and Walter Koenig. The differences are not something that can easily be explained by the Narada's impact on the timeline.
We don't know what the characters were like in 2258, and some of the observed differences are probably more attributed to the acting talent playing the roles.
If Starships have evolved in a different direction, it is possible that Starfleet Training may have been subtly changed between 2233 and 2255.
Delta Vega is near the galactic perimeter in TOS and near Vulcan in XI.
There are 2 planetary bodies named Delta Vega.
In TOS, starships are a rare commodity, so much so that an object could leave Klingon space headed for Earth with only one Federation starship in a position to intercept and that one starship isn’t even ready for launch. In XI, the Federation is able to gather a TNG-ish 40-vessel fleet at Vulcan in almost no time.
It is not established that 40 vessels are sent to Vulcan, but there are around 7-8 ships. Starfleet obviously has a fair number more ships, and in TMP, we don't know where they are. It doesn't mean they don't exist.
At least five (Farragut, Walcott, Hood, Antares, and Enterprise) are on Earth and ready to go. Similarly, the Klingons are able to muster a 47-ship fleet to confront the Narada.
It's possible, since Rura Penthe is a Dilithium mining facility, that a large number of Klingon ships may have been in orbit gathering Dilithium, and perhaps guarding the captured Narada.
If the Narada comes to life after 25 years with those missiles, there goes the fleet.
Pike appears to be much older in XI than in TOS, despite XI being set nine years before The Cage.
The Cage was set 17 years prior to the Enterprise's first year under Kirk in 2266, which would place those events in the Talosian transmissions seen in The Menagerie in 2249.
This allows 6 years for Pike to go from mid-late 30s to early 40s.
The Kelvin has escape pods with which the entire crew is able to evacuate the ship.
Actually, the Kelvin has a large shuttle complement, and no mention is made of Escape Pods. If the ship was, as per the web site, a Survey ship, she might have a high complement of shuttles for hands-on planetary surveys.
Federation ships in TOS apparently have no such capacity.
Shuttles in TOS? Not on Constitution-class vessels. However, we don't know about ships from 2233, or whether other classes exist in 2255/2258 that would have a large shuttle complement.
In TOS, Chekov is 22 years old at the time of Who Mourns for Adonis. In XI, he is 17. But XI is set nine years before WMFA. (Maybe it’s possible that the Narada's intervention caused him to be born several years earlier than in the original timeline, but that seems like a stretch.)
This is more of an issue, but since Chekov was born after 2233 in both realities, conception point may have changed. It's as simple as hearing about a friend on the Kelvin changing their mood.
In XI, Scotty has a tribble at his post on Delta Vega. In TOS, removing a tribble from its predator-filled natural environment is a disaster.
Tribbles are certainly known about, and it's possible that a Neutered one was picked up along the way.
We also see Tribbles in a bar on Earth in Star Trek III.
Can all these differences be treksplained by the Narada’s intervention, or are they not required to be?
They can, for the most part be
indirectly explained, via the Butterfly effect or a little imagination.
In practical terms, however, an explanation is not really needed.
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