From the Player's Basic Rules, page 71 (or PHB p. 191):
In contrast, you can move through a hostile creature's space only if the creature is two sizes larger or two sizes smaller than you.
An ogre is a large creature and elves are medium. So an ogre can't move through an elf's space. In your previous example, if we have a 10-foot wide passageway with two elves standing side by side, the ogre can't get through without killing one of the elves or forcing them to move.
The rules on squeezing into a smaller space are for an ogre trying to move along a 5 foot corridor. The rules on creature size that you've quoted back this up—the ogre isn't actually 10 feet wide, that's just the space he controls. So he can move through a 5 foot wide gap, but it's cramped and he can't move freely.
Now, you might be thinking that it's a bit unfair on the ogre if the 2 elves can form an impenetrable barrier against it. As you've said, he'd rather shove them aside than squeeze between them. And he can do just that! From the Player's Basic Rules, page 74:
Using the Attack action, you can make a special melee attack to shove a creature, either to knock it prone or push it away from you. If you're able to make multiple attacks with the Attack action, this attack replaces one of them.
The target of your shove must be no more than one size larger than you, and it must be within your reach. You must make a Strength(Athletics) check contested by the target's Strength(Athletics) or Dexterity(Acrobatics) check (the target chooses the ability to use). If you win the contest, you either knock the target prone or push it 5 feet away from you.
So you're right, shoving is the ogre's answer here.
Now, with your new example, (2 elves in a 15 foot corridor with a 5 foot gape between them), things are different. The ogre does indeed have to squeeze between them. Fortunately for him, it's not going to matter all that much.
Why? Because he's almost guaranteed to be moving on his own turn. Unless there are more enemies than just the elves, he's not going to provoke any opportunity attacks while squeezing, and he can attack before or after he squeezes. So the only squeezing penalty that is actually going to apply to him in this situation is the double cost for movement.
Note that if he stopped between the elves, all these penalties would apply to him. That makes sense though—standing between two enemies with not enough space to move around in would make it difficult to dodge attacks or attack effectively.
This is one of those edge cases where a DM would need to make a ruling.
Given that the Tarrasque is exceptionally large, it would be reasonable for the line
This reduction decreases its size by one category
to take precedence over the line
The target's size is halved in all dimensions
In other word, the Tarrasque is reduced by more than half such that it can be considered a Huge creature, rather than still a Gargantuan creature. Given that magic has been used to reduce the Tarrasque (meaning that presumably a resource has been expended and the Tarrasque has failed a save, etc), it seems fair for the Tarrasque's opponents (presumably the party) that this should be the case.
Of course, it may also be reasonable for a DM to say that the Tarrasque is so large that reducing doesn't reduce its category, so the lines above take precedence the other way around, leaving the reduced Tarrasque as still being a Gargantuan creature.
Ultimately, without a Word of God answer, this one is up to the DM.
Best Answer
Space doesn't only mean the area you physically inhabit in combat
An ogre isn't 10 feet tall by 10 feet wide, they simply command a 10 X 10 combat square area. It would have a penalty to Dex moving through a 5 foot wide square because it can't effectively dodge when the walls are brushing up on it's sides.
This is the same for Medium and Small creatures, who command a 5 X 5 square. They don't command that area because they're that big, they command it because they're actively engaged in combat inside of it and moving within it.
Basically, you've equated combat squares with obstacles to movement in general.
For example: A space large enough for a medium creature to move freely through would be a standard door. However an ogre would need to squeeze through because they're wider and taller than a standard door.
So when you're looking at a medium creature squeezing through a small creatures opening, it's akin to an average person trying to move through a crawl space. While the small creature would be able to do it easily, the medium creature would need to squeeze.
For example, I ran a game with a bunch of small creatures pestering the party and retreating to bolt holes with an opening that would only accommodate small creatures. That means the opening isn't 5 feet wide. By squeezing rules, a party member of medium size could have traversed the tunnels, albeit slowly and at significant disadvantage. Whereas the enemies could freely move and attack using the tunnels for cover and being an extreme and persistent annoyance.