I'm a very long time Mackelroy (various spellings) researcher. It is good to see another cousin.

My ancestry goes along the line of the compiler of Family Adventures you referred to in your website.
John and Frances Mackelroy
Archibald Mackelroy
Avington Muckleroy
Isaac Muckleroy
Elijah Muckleroy
James Muckleroy
James H. Muckleroy
John B. Muckleroy
Gerthie Muckleroy Davis
Betty Davis Stokes

The Family Origins book has been microfilmed by the LDS Church and can be ordered on loan from them.  I do have a copy of the book and helped the author, David V. Muckleroy, get started on his research back in the 1970s.

About the source of the name, my cousin was visiting her son who was working in Scotland a few years ago and was sight seeing.  At one of the "sights" she mentioned that her Mother was a McElroy and he corrected her and said that the name was Muckleroy.  Since that was what her mother's name actually was, it made more sense.

The first spelling that early researchers found was Mackelroy/Mackleroy.  Of course there was even one spelling Mackelduroy/Mackleduroy (I forget whether it is el or le as it was/is not consistent).  Since they mostly didn't read and write, they were at the mercy of the clerks, etc..

I have heard stories of our ancestor being a Scottish Chieftain's son who preferred to be a highwayman - and was shipped off to the colonies.  I have no idea if there is anything to it or not, but it is a fun story.  I think the story comes from some Scottish games folks who are into tradition of things Scottish.

I have collected a bit on the Sims family.  However, as I am not from William, I do not have everything as documented as I do on my branch.  Still, it helps as my bunch traveled with your bunch until they left Oglethorpe Co. and some, even then.  There is a genealogy of the Sims Family that has a gedcom on Ancestry.Com.  What I know of that coincides with the information on the gedcom seems to agree.  Other than that, I cannot say.  If you can't find it, I can send it to you.

I'd love to hear from you.

Betty in Houston