.

So, elves. Which elves are you referring to? Name your source: perhaps the Elder Edda? The Silmarillion? Dungeons & Dragons? Or your own creation?
Even if you answer "I MEANT THE CANONICAL ELVES," still provide the most suitable example, or even examples, of similar elves.
Without providing an example, arguing about elves that only exist in your head is pointless.

Why delve into such nonsense?
And which knights did you have in mind??? The Battle of the Ice? The Pelennor Fields? The Crusades and other such nonsense?
In a PvP match, my level 90 elf will crush your knight from Age of Empires with just two spells!

Then, about mounted knights with crossbows. Why not? It's quite possible. Only, such a variant was unlikely to be widespread. Knights are heavy cavalry; their functions are very different from those of support troops, that is, archers and crossbowmen. They don't really need a crossbow. Although, if desired, a squire could follow the knight, ready to hand him a crossbow if he wishes to shoot, just as the squire himself could use it if necessary.

You asked the question, you answered it.

Lord mentioned that knights were required to be proficient with a crossbow. Again, this is quite possible. But they most likely used it more for hunting than for military purposes.

"Proficient"? But any peasant can handle it, so what's the point of them being proficient? To carry it around for show, of course, why not? It looks stylish. To use it on horseback in armor? For hunting? Hunting in armor...? Any of the "crossbow + horse" options doesn't really go with heavy armor; it's only suitable for shooting wild boars on a hunt or showing off to the ladies.

Reloading a crossbow while mounted. The crossbow with the crank mechanism that you provided as an example of a medium one actually belongs to the category of heavy siege crossbows. These were rarely used by field armies, only by besieging forces or the garrison defending against them.

Well, how should we call those Chinese things mentioned two paragraphs above? Super-heavy? Yes, you probably wouldn't carry that around. But the one I mentioned is just right.

Let the knight take a small crossbow, demonstrate extraordinary skill, load it, aim, and fire. WHY THE HELL?
You might as well load catapults with dogs, right? The dogs will fly and bite the enemy, and half of them will end up as a bloody mess. Demoralizing!

At a minimum, it absorbs the energy of the arrow. So why make and wear chainmail, which costs ten cows, which requires a lot of time to make, and adds ten kilograms of weight?

Then why do you need archers if they're useless? Did everyone just want to show off with ranged troops? Which, by the way, cost much more than ten cows to train.

If knights crush spearmen, break through any shield walls, crush archers (with a crossbow, lol), and don't even pay attention to peasants... then why do you need any other troops at all? What about tactical maneuvers, skillful formations, and well-placed archers? Why bother?