I can answer the "why exactly is she with Oliver Queen?" question. It's all due to two factors.
1. Judd Winick drove all of the dedicated Green Arrow fans off of his own book. DC needed to do something to win back all the fans who could remember back to the days when Green Arrow wasn't a complete moron and a womanizer (i.e. The Kevin Smith run) and they somehow felt that marrying him off to his old girlfriend would do it.
2. Someone said "Marvel is having an extremely ill-advised and nonsensical wedding in one of their books that is getting a lot of press - we should have a nonsensical wedding too!"\
For the record, I love Ollie and Dinah as a couple. I just really wish the wedding and everything leading up to the wedding had been handled better.