But since one of my players have selected a hand crossbow build, going for the crossbow expert and sharpshooter feats, I really need to sort this out for myself.
I have found several threads, but decided against posting in any of them, since I really am not arguing against anyone in particular.
Let's see if I got the basic facts straight:
1) The first (of many 
2) Regular crossbows (light and heavy) are two-handed while Hand Crossbows are not two-handed and instead Light.
But none of that matters:
3a) the errata on Ammunition says "you must have a free hand to reload", is there any distinction then of a weapon also having the Two-handed property?
You would think it would make regular crossbows impossible to use for serious fighters (since, being two-handed, you wouldn't have the free hand needed to reload them). But then they go ahead and errata Two-handed: "This property is relevant only when you attack with the weapon, not when you simply hold it." Beyond the inelegant inexactness this would indicate you could indeed hold it with one hand while the other reloads it!?
3b) the Light property of hand crossbows is also completely inexplicable, since 3b-I) the rules on two-weapon fighting specifically allows only melee light weapons! 3b-II) the crossbow expert feat allows the equivalent of two-weapon fighting for hand crossbows specifically rather than "all light crossbows"!
I can't understand it. It seems as if despite the differences in "looks" and in properties, all crossbows operate identically:
Despite the hand crossbow being light you can't actually dual wield it (rules on two-weapon fighting says "melee")?!
Despite the hand crossbow not being two-handed you can't actually dual wield it?! (You can't reload if you hold two one-handed weapons, but you can if you hold one two-handed weapon)
4) There is a fighting style available to select classes called "Two-Weapon Fighting Style" you would think is aimed for the character that want to dual wield. However, the Crossbow Expert feat provides the same benefit (being able to add your Ability modifier for the bonus attack)! So why would you ever take this fighting style?
Indeed, ranged weapons (meaning crossbows) do the same damage as melee weapons. But you can take the Archery Fighting Style without losing that second attack ability modifier since you get that from Crossbow Expert, effectively letting you "stack" two fighting styles.
But it gets even worse: the one and the same feat also voids the main disadvantage for ranged characters; getting disadvantage once you're swarmed!
Then it feels like a slap in the face when they start their sage advice with
What the hell? If anything, the rules specifically and directly converge on one thing and one thing only: turning the hand crossbow into a semiautomatic weapon!! 
In the end analysis: what were they thinking?!
It appears that:
* the fact that hand crossbows are light and not two-handed is actively stopped from having any game effect. Indeed, not only do you gain the exact same rate of fire with one crossbow, you are specifically forbidden from using two!
* there is absolutely no upside to melee dual-wielding?
Please tell me the WotC devs have acknowledged their rules suck balls and that they are actively working to straighten out this complete and utter mess! Please!
Or at least tell me they have explained WHY ranged two-weapon fighting needs to be strictly superior to melee two-weapon fighting? (Keep in mind that I haven't even started on the fact that the perhaps most powerful feature of all in the entire PHB is denied to dual-wielders but not to this ranged build, namely Great Weapon Master/Sharpshooter!)
There must be a clear reason or they wouldn't have engineered a ruleset with such counter-intuitive results, where almost every other option is better than two-weapon fighting, right?
Right?
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