Thanks Agha,
The thread you refer to is referencing my Robert. Jim is a distant cousin to me. He, and another cousin, have cobbled together various documents and information, and I have tried to track it further. I have been unable to contact Jim for the last couple of years.
I have Margaret Sumerville's marriage cert, and I can't remember what else

, don't forget I am from home just now. I haven't touched this line in several years, as I was told the records were destroyed. Now it turns out that this is not necessarily the case, and that some are even on line. Since last I looked at this line, I sustained a brain injury, and I am concerned with not giving incorrect information to you, and sending you haring off.

I do have my software with me, so everything I am giving you is coming from there. I am missing about 2 file cabinet drawers full of photocopies.
I think the only other Somerville doc I have is Robert's marriage to Ann Gillies in Co. Durham (21 Jun 1866), Collierley (Dipton).
Specific questions you posed:
I learned online that Clonavaddy was a registration area, not necessarily a townland, so when someone gave their birthplace as Clonavaddy, that could cover a wide area. This info came (I believe) from Robert's marriage certificate. I really can't remember where the date came from right now.
Margaret's parents. When I received the Somerville docs, I believe I was told that she was cousin to George (no docs to prove, though). I dimly recall seeing their names on the marriage certificate, but I have that feeling in the back of my head that something at the time didn't make sense. It might be something as simple as seeing Mary Somerville Moore as the mother, and wondering if someone made an error. Obviously, it could be accurate, I just think I noted it for further research. (Sidebar: this wariness comes from a 2nd cousin, whose family was unaware that she was illegitimate, as her death certificate listed a man that I knew to be her grandfather as her father. Even her surname was wrong, as her true father had acknowledged her. When they changed countries, her mother used her own surname. When the daughter married, she gave the name she was using. I have the certificate, and it's wrong. They had constructed a story and stuck to it. Add to that some of the errors on census records ... )
That reference to the Carnteel pub is the second I have seen. I will try and check it out.
With me being so far from my records, I would be MIGHTILY GRATEFUL for anything you would care to post.
I would also like you to tell me what search parameters you used on the LDS site. I have searched there before for my Somervilles and found almost nothing. Did you search from the main search screen?
Thank you very much for all if your help.