I can only answer #1 from having just done something similar. I assume you are talking about the Preferred Seats that show up in blue on the DL seat maps? As you stated, those are only visible to anyone holding status with DL (or other sky team member, not sure on that) and should be free for you to book.
I would just open up the seat map under your Skymiles account and check out which seats are available together, then call Delta and explain that you are on separate PNRs and ask them to place you together in 15A and B, or something like that.
I've done this recently, my father a PM, myself an FO, traveling with my mother and two sisters PIT-ATL-GIG. We were on three separate PNRs due to varying return dates, so the way I got us into the preferred seats was just to call the PM line and kindly ask them to move us together into the preferred seats. They had no problem helping me out with that.