You are absolutely and entirely outside rules territory from the moment you've decided to fire someone out of a trebuchet and into the ground, so only loose inferences from the rules are going to help us here. But let's see what it gives us to work with.

Forewarning: if you're after real-world simulation in D&D 4e, you've come to the wrong town. The designers consciously eschewed real-world simulation in order to just make a fun and well-balanced game, so 4e runs on simplistic game logic that laughs in the face of our Earthen physics. 4e has squircles for instance: consider that the circle is defined as a shape where every side is equidistant from the center, and that this perfectly describes a square room of any size as far as a walking PC is concerned. Instead of attempting to draw real-world physics out of D&D 4e—which 4e will either directly contradict or simply not help you with—use the game logic, leave the calculator aside and use your time to beat stuff up and do fun things instead.

this would launch him at about 3X the speed of a normal fall (150 feet per second instead of 55 feet per second).

You have some parts of this conclusion wrong.

First, there's nothing that makes him fall faster. You have a trebuchet, but we don't know how fast he's actually traveling: D&D 4e doesn't exactly have specific rules for trebuchets that I'm aware of, unless you'd like to point us to them.

Second, the speed of a normal fall is 500 feet per round, at the end of the creature's turn (Rules Compendium, p209, High-Altitude Falls). Since a round is 6 seconds, that's actually 83.33 (recurring) feet per second, and nobody ever falls faster than this. That's far less than the falling speed of a skydiver (who is trying to fall slowly), so we can assume accurate simulations of Earth physics and wind resistance are to be left well aside: things work differently here.

So. Your orc hits the ground normally, and takes the normal amount of fall damage.

If they hit the wall instead, we don't know if they're really traveling fast enough to even simulate it with an equation that works from 500 feet/second of fall damage. You're in house rule territory, and we've established D&D 4e doesn't really care about what physics has to say about falling, so make a judgement call with a small portion of fall damage, or decide your character survives it fine because they might as well.

Alternately, whilst I would not advise against firing D&D 4e characters out of trebuchets on account of how awesome that is, I would at least just advise saying they succeed and skipping this trouble.

Unless they're an enemy NPC, in which case I advise they die hilariously.

there's nothing in the rules telling you things you're holding don't get helped by slow fall.

So to get this out of the way, rules as written say absolutely nothing about when you stop falling if someone catches you, much less how to catch someone that's falling.

We will just skip to the part where the monk and lidda are falling to their deaths as one mass. Our monk buddy is going to cancel the fall damage just fine due to his class feature (though one could argue he needs a free hand or something to touch a wall depending on the world). Lidda also gets off easy, so long as the monk keeps a hold of her. From a rules perspective, anything you're holding is usually part of your inventory and the class feature includes the halfling our monk is holding. I certainly don't see any part of the rules demanding that you make saves against the thousand gold pieces in your pockets ripping the muscle off your skeleton when you hit the floor, so a halfling is no different in this case.

Of course, more realistic GMs are welcome to give the monk a pass, and hope the halfling doesn't rip his arms off when they land. She is an object with mass and momentum, so if it's your idea of "cool story bro" for the monk to potentially lose an arm knock yourself out. There's nothing telling you no either way.