impart your knowledge unto me, trader sages..

1- It depends on the game, if the it was bundled or if was bundled in a tier 1 or was a humble montly, or maybe if the game is awesome and everyone wants it (supply and demand)
2- you can't ?(do you mean cut 1 single key in more than 1 key? or delete a game for ever of your library?)

You can't ask Steam to 'remove' a game if you bought it in a Steam bundle or when 1 key activated multiple games, if you want to get rid of 1, you need to get rid of all of those, it's to bad!

I’d suggest moving this from deals to off-topic.
1 - I’m not sure. You could guesstimate, but it would be some work on your part. For example, let’s say you have a bundled game that’s trading for $1 worth, you might ask for cards that give you that much value. But that would be pretty involved on your end. That said I’ve definitely seen it done on steamtrades, so there may be a better system than I’m aware of.
2 - sadly no. I’ve cleaned up my library a few times and ran into this. The best you can do is hide the game. It doesn’t remove it from your game count, but takes it out of your general library. You can see all hidden games once you have one (kind of similar to categories, if you use those)

Cards have a lost a lot of value vs when they were first launched. With more finely divisible currencies than USD or EUR where a card is at min listed for $0.03/0.03€, you will see it's often only a bit more than a cent. Assuming you want a decent game and the other user isn't (for good reasons) interested in buying cards at their full market valuation, you could be trading a hundred cards of some GoGo bundle level trash games for a decent game. Seeing how you want to delete good games from your account, I doubt you're interested in the sort of game keys that are only worth a few cards.
Of course there are still highly valued cards, commonly when the game has some very valuable backgrounds or emoticons, and for many but not all AAA games.
Personally I'd only take cards as tradables if they were somewhere in the range of 30-60% below market. Unless they're cards hand-picked by the other user to craft badges, they'd also want to sell them, and have all the effort of listing and confirming.

I haven't traded card for games, but from what I have observed, the cards will be priced at the highest buy order minus fees, and the trader will usually ask for the high value cards to recover money faster with less cards to sell. They may not take 3-4 cent cards, as those have low demand, as reflected in the price.

I think you've received the answer for your question 2 already.

Now, for question 1, I do accept trading cards for my games. I usually ask for a 25% markup from my PayPal price (before the market fees). For example, a $1 game, would be $1.25 for cards. I also make the person sending the cards make the trade offer, it's more work than I want to do. However, you can use this website to help you out https://steam.tools/itemvalue/

They provide you with your inventory value before and after market fees.

Now, some traders say just a flat amount of cards. It all depends. I personally prefer PayPal or even wallet, but I'll accept trading cards.