Maro *coughs* Feel free. I'm about to rescue Leon, I'm just being lazy about it. :p
YES YOU CAN DATE LITERALLY EVERYONE AT THE SAME TIME IF YOU WANT.

Well, all of the 6, but yeah. As long as your friendship level with them is high enough. It worked at 7 when I asked Vishnal, but I had to get Dylas up to 8 before he would date me. I think I have to get Arthur up to 8, too, the way it's going.
And yeah, you have to get through the something important & then the side event. Which also, if I read the wikia correctly, when playing as a girl, you don't have to have the ring and all that unless you rejected their proposal at first and then you go and propose to them again. Which is kinda awesome. I've gone on 3 dates with Vishnal and one with Dylas.
Also, I know the feel about being lazy.

I just tell myself it's because I want to get all my friendship up. Yep...that's it... *coughs*
@Mika Oh wow, that's awesome. And 1000+ level monsters?

Yikes!