Small creatures can't move through tiny enemy creatures' spaces
Yes, your reading is correct. A medium creature can move through a hostile tiny creature's space, but a small creature cannot (without using optional rules, like tumbling or overrunning [DMG, p. 272]).
In some cases, this makes more sense than others. Keep in mind that small humanoids are about half the size of medium humanoids, and about a quarter their weight (PHB, p. 121), making them roughly the size of a five year old human. Even though these humanoids can be extremely strong, they also are light enough that a relatively small amount of force could reposition them. Certain tiny creatures, like a full grown badger or a hawk (or a Crawling Claw which could carry things twice the weight of an an average halfling), could reasonably impede their movement.
On the other hand, "tiny" is a size category that applies to creatures of quite a variety of sizes. This is a necessity of the rules, since "tiny" is the smallest size category and will apply to anything which is smaller than "small." By way of example a cat, rat, and (normal) spider all all tiny sized creatures, even though these creatures outweigh each other by a factor of 20 (20 spiders to a rat, 20 rats to a cat). As such, a DM might make a ruling in the case of specific tiny creatures that ignores the usual rules on movement (such as deciding that a spider can't impede the movement of a small creature, but a hawk can). However, by the Rules as Written, a small creature cannot move through any tiny enemy creature's space.
The Wyvern is unable to pass through the Sorcerer's space and must move around them or attack them.
It would seem there are no additional rules that come into play here, either from invisibility or movement, so unless your monster has a trample ability, it would not be able to move through the Sorcerer's space, as per your quoted movement rule. (It feels a little weird that your Wyvern can't move over a weedy Sorcerer, but imagine it was trying to bypass a jacked-out Orc Barbarian who is also medium size and probably trying to defend the Warlock, and it feels like a reasonable rule.)
So what should the Wyvern do?
Firstly, it moves up to the Sorcerer and "can't enter that space". If you were describing this to a player, you might say they collide with an invisible force. Your player wouldn't do any damage for accidentally walking into an invisible monster, and neither does your Wyvern, despite the size difference.
Now; moving in 5e can be split up in as many ways as you like, so at this point the Wyvern has a number of options:
Option 1: "The Wyvern guesses the invisible force is the Sorcerer that just disappeared." If the Wyvern's been paying attention, this could make sense, and if the Wyvern considers them a dangerous target attacking them again is reasonable - Attack the invisible Sorcerer at disadvantage. (Feels harsh if your Sorcerer was really trying to avoid the attack, but you'll teach your player to also get out of the way.)
Option 2: "The Wyvern has no idea what the invisible force is, and tries to shove it aside." - Use the Shoving a Creature rule. If you successfully shove them out of the way, continue moving on to attack Warlock if you have any attacks left.
Option 3: (If you're not fighting in a 10ft wide corridor). "The Wyvern bumps into something, and like you would do in a frantic fight situation, just continues around it." - Move around the Sorcerer, potentially provoking an attack of opportunity (which, if the Sorceror makes, your Wyvern could then just turn around and bite them; remember that it can always break up it's movement in response to the situation changing).
If you are in a 10ft wide corridor, then you'll have to pick one of the other two options, either using attack actions to shove the invisible player until you can reach the Warlock or eventually deciding to attack the invisible player - depends how stupid your creature is.
With my players, I favour Option 3, but if you want to teach your players that Invisibility isn't Invulnerability (as some players seem to think) go for 1, and yours will soon learn to also get out of the way.
Best Answer
Forced movement can force creatures together
The restriction against moving through a hostile creature's space only applies to willing movement. That is, it applies to movement in the sense of "on your turn, you can move a distance up to your speed" (along with other willing movement mechanics such as readied moves). There are many ways other than movement to change location, willingly or otherwise, and some of these can force a creature into another creature's space. In short, this rule is irrelevant when considering falling.
There aren't actually any rules for falling in the PHB
That might sound like a surprise, but here is the entire section on falling in the PHB/Basic Rules:
This section describes what happens when you land at the end of a fall, but it doesn't specify when, how, or where you fall. That is left entirely up to the DM. XGtE adds some additional rules for falling, but doesn't really take any control away from the DM. For example, the rule about rate of falling doesn't specify that you fall straight down, so the DM could decide that your fall follows a parabolic arc if that makes sense for the situation, just as easily as they could using only the PHB.
So what about falling into a creature's space?
This leaves us with a bit of a conundrum when looking at the rule you quoted from TCoE for falling onto a creature. This rule says what happens "if a creature falls into the space of a second creature," but there's no falling rule in any of these source books that specifies how or if this can happen. However, based on the fact that the subsection is called "Falling onto a Creature", we can infer that the likely intent is that these rules should apply to any creature falling onto another creature. In other words, the TCoE rule appears to assume that when a creature falls onto another creature, the falling creature lands in the second creature's space and triggers the rule.
Ultimately though, the rules for falling are left mostly in the hands of the DM, who should make a ruling appropriate to the situation.