Hi Christian. I would answer:
1) Soft play (you mentioned slow play but I think you mean soft play), is when you have a dominant hand and don't act with it in certain circumstances. That isn't what's happening in your illustration.
2) In this case, the player doesn't have a strong hand (just the opposite) and is folding to save himself some money (he thinks)... because he has a rag hand and was bluffing when he made his bet. He's facing a bet and he folds a rag hand, that's not soft play (normally). Now, if he has one chip to call to take a risk that he has a hand, well maybe he should call but IMO that's probably not penalizeable under the soft play rule if he has a truly rag hand... it just depends on each case as the rule is currently written.
3) Here, I think the dealer should accept this player's fold as binding when declared because the player was facing action, the hand is instantly dead due to a fold while facing action and cannot be re-activated under any circumstances.
4) Now this still leaves the question of collusion, which as you say isn't likely. But the house has the authority to check the cards to confirm no collusion is involved (or any unsolicited "charity" for that matter).... i.e. the player tossed a rag hand. In this case you confirmed in fact that's what he had, there's no case for any further action against this player, IMO.
This all said, the soft play rule can be applied by a TD when they see what they consider to be softplay.... the rules as currently written allow for TD judgement IMO.... Personally there's only one move that I automatically deem to be soft play, everything else is based on the circumstances. Thanks for the post, numerous people have questioned the soft play rule in various threads.... and perhaps thats a subject for review at the next TDA Summit.