The first option
In my opinion, the first option is the most likely interpretation of the rules as written, probably what the authors were going for, and the best way to run things at the table.
Multiweapon Fighting does exactly the same thing as Two-Weapon Fighting, it just affects more weapons. So if you have more than two weapons, you use Multiweapon Fighting instead of Two-Weapon Fighting, but they amount to the exact same thing.
The game works better if you basically pretend they’re the exact same feat; the distinction is only made because Two is such a common default, and Multi is usually reserved for more monstrous creatures (though there are plenty of ways to access it as a PC).
I want to challenge your approach to this. As you note, the Light property is defined like this:
Light. A light weapon is small and easy to handle, making it ideal for use when fighting with two weapons.
This property isn't just there as a computer function — it's also part of the basic description. A hand crossbow is small and easy to handle. And it is, as far as crossbows go, ideal for fighting with two weapons. You may need special practice and training (as represented by the feat) to take advantage of it, but it's undeniably better than any other crossbow for that case.
Is it true that mechanically there are no current rules which make particular use of this property? Meh. Probably so. But it does not mean the property shouldn't be there, because that kind of mechanical interaction isn't the only reason weapons have descriptions.
It might have been more elegant from a "the puzzle pieces must fit together!" perspective if Crossbow Expert said "a crossbow with the light property" instead of "hand crossbow", but I don't think that really would add much of actual value.
Of course, there's always the possibility of future features which apply to "light ranged weapons" or the like.
Best Answer
The feat Double Slice allows the attacker to add his full Strength modifier to the damage of an appropriate off-hand weapon even at range
The benefit and normal entries for the feat Double Slice say
An appropriate off-hand weapon is either a thrown weapon or a specially built composite longbow, specially built composite shortbow, or sling:
Let me make it clear that the above is my reading of the feat, based on the feat's normal entry, and, really, sometimes a feat's normal entry is the worst thing about a feat. So it's possible a more generous GM may allow an attacker to add his Strength bonus to the damage dealt with his off-hand light crossbow (or pistol or bola) while receiving no Strength bonus to damage with his primary hand light crossbow, but—and I'm gonna go out on a limb here—I suspect such a GM will be in the minority.
The benefit of the feat Two-weapon Rend when used with ranged weapons will depend on the GM
The benefit of the feat Two-weapon Rend says
So, yeah, that's actually pretty okay with a pair of thrown weapons that deal damage plus (either whole or in part) the attacker's Strength bonus like, for example, daggers or—I don't know—spears or something, but it gets a little weird when going all The Killer with a pair of hand crossbows and absurd when going all Spider-man with a pair of bolas. In any case, before you bring this to the table, ask the GM.
For example, this GM has no problem with an attacker who hits the same foe first with a primary-hand dagger then with an off-hand dagger to then deal the extra damage from the feat Two-weapon Rend just like it says. However, this GM would struggle to allow the same attacker getting 1½× his Strength bonus were the same series of attacks made with, for example, a pair of light crossbows—but as it's really difficult to maintain such a rate of fire in the typical campaign and an attacker would've devoted so many resources to doing this once per round, I might also find it difficult to say No.
These feats are unmentioned by the FAQ, and I couldn't locate any developer commentary on this. In fact, on Paizo's messageboards, this topic is rarely discussed at all (here's from 2010 and some crazy stuff from 2009). I suspect this is because both two-weapon fighters and thrown weapon warriors have better ways to spend their precious feats.