You were wondering about the source of the "King" middle names. I can't really help apart from to point out I have "William King Stevenson" in my family. I believe his mother was Elizabeth King, and I'd guess her father was William King (Co Donegal I think). People quite often gave the name of the mother's father to the eldest son.
I came to this thread because I have links to Osbornes of Altmover and Fergusons of Cookstown area (and McCauslands of Limavady). I've only really skimmed through the thread, so I'm not sure whether or where we join up.
I think you said you've been to PRONI and you picked up a few dates, but your not sure of some if they're your family. I wonder if you've scanned through the BMD notices on the Belfast Newsletter? You mentioned Ancestry.com, so you probably have a full subscription - they have scanned images of the Newsletter up until about 1925 (with only a few small gaps):
http://search.ancestry.co.uk/search/db.aspx?dbid=2193&path=Don't rely on the searches from Findmypast, The British Newspaper Archive, etc, because they don't pick up everything (nowhere near as good as Trove in Aus). If you know the exact date, just find the next day in the Newsletter and work through up to 2-3 weeks after the date. Not every family posted BMDs in the Newsletter, particularly not in the early years, but I think it's worth a try.