Re: Atomic swap? Objection: I don't think it means decentralized coin exchange.
I think not, because you are dealing with a complete stranger- perhaps an adversary. So you must validate for yourself that you have e.g. 1 confirmation and you really can't trust any external party for that information. There is also the question of adversarial forks, so this would be the only way to guarantee you are running on the chain you think you are, and that your trading partner is not trying to use a slightly different chain. But I'm sure these services will be made available anyway, and people will use them without thinking about it 
So I interpret that it would be "possible" but potentially insecure. Well, for small amounts of less than (let's say) ~$100 (I think those make up the majority of altcoin trades) it would be a relatively "doable" solution if a swap-capable client checks various block explorers of different origin if the confirmations have happened.
Yes I think just like some merchants accept zero confirmations for coffee purchases, people can create XCAT systems that allow the user to take greater risks. But in XCAT systems your trading partner is likely an adversary..the relationship is the opposite of doing business with Bitcoiners in person, where we are excited to meet and do business with each other.
You'd probably only be able to trade with other people who have the same pruning settings on their chains, but this is just a wild guess.
Interesting, but I "don't get it" for now. Could you clarify why you think the pruning settings should be similar or give me a hint where I could research?If you prune to 550 and your trading partner prunes to 950, then she has a 400 block informational advantage over you. If she is able to introduce a serious network disruption (perhaps a hardfork attempt involving a lot of hashpower!?) during the trade she might be able to take your coins while giving nothing in return.
I would personally want to make sure that I'm not risking anything if some kind of serious instability arises, especially when trading larger amounts. So in many cases that means no pruning at all.