On the $4100-ish biz-class fare, it's hard to say because I'm not familiar with prices out of DAY. If you think the price is good, and you want it, then I don't see the problem buying it. One thought is if you want or can travel over Thanksgiving weekend, there are likely to be very attractive biz-class fare on sale in about one to two months. Just monitor the premium class mileage run forum.

Second question sounds like you want to do a Same Day Change on an international flight. You can't do that, but it is theoretically possible an agent might allow it if you ask. Personally, I don't see a six-hour connection in ATL as so horrible. You can eat a leisurely lunch at One Flew South if the Skyclub does not appeal.

Correction: Houserulz77 is correct. I thought you wanted to SDC in DUB. You probably can SDC in ATL. A few agents might be overly strict about the prior international leg and the rule interpretation; if so you can try again at another Delta service center or at the departure gate.