Okay, question about Steam coupons!?

I think the Steam coal event did let you craft coupons.

Other than that there are also Steam keys that will give you a coupon upon activation (sometimes Alienware gives you such Steam keys).

Sometimes coupons seem to appear out of the blue, but AFAIK it turned out every time, that they are given to users for already owning certain games when the devs decide to hand out these coupons.

And ofc - as you already mentioned - crafting badges gives you coupons, too.

I've never heard of coupons for achievements, purchases or other random coupon drops either.

There've been coupons as part of bundle deals/preorder bonuses etc. This is because the developer/publisher can control if, when and how they want to have a series of coupons distributed.

I don't know answers for your questions, I just wanted to use occasion, maybe someone will know - is it possible to predict what coupon will be droped while crafting a badge? For example - when I craft a badge from game XXX will I always get a coupon for game YYY or can it by any coupon from some pre set base?

As for ORION: Prelude - just buy it. It's extra cheap. I don't remember this game being in any bundle, but as far as I remember it's discounted on every steam sale. Also maybe try to trade it for some cards, this game was given away free, maybe some people still have keys.
Also, honestly I think that ORION is too cheap to be in bundle. Devs fixed the game, gained player base, they convince people to play it. I think they are already in the point where they wanted to be, there is no reason for them to include their game in any bundle.

Never heard about 2.

I've gotten random coupons once or twice

1. Events don't drop coupons any more since Steam events have regressed to the two season sales, when coupons are replaced by event cards.
2. Never heard about achievement-specific ones. If they exist, I assume they are related exclusively to Valve games.
3. Some, very some, and only in times when the dev/publisher has it. Those coupons are a light version of the 2/3/4-packs: buy the game, get a coupon for it so you can buy one for a friend cheaper. Sometimes bundle sites have similar promotions; Humble Bundle just had one a month or so ago I think.
4. Yes, this is true. Exceptionally rare though. It may be tied to a dev/publisher just asking Steam to spread a few hundred thousand coupons, but that is only my guess. I have no idea what causes this.

They gave 75% Warhammer coupons for random people about a month ago.
And I think purchases would mostly give you a coupon when making the purchase during an event.