Today while reviewing a DNA match, I made an amazing breakthrough and was able to add several ancestors!
My notes indicated that Isabella's father was listed as Edward on the marriage record I and that he was an engineer. Big problem here, because I can't find the marriage record I was referencing. I think maybe I ordered a copy and it was too large to scan easily so it wasn't scanned and added to her images in Legacy. It is unlikely I would have viewed the full marriage record online somewhere and not copied it into the notes at least, with the URL. Only other possibility is too horrible to consider, which is that I viewed and attached a marriage record at Ancestry.com, and some time since then they removed that database and my source link to it.
I was able to find an Isabella Nelson born about 1838 in Scotland, with a father Edward in the 1841 and 1851 Scotland Census. It so happened that the location matched up perfectly with the DNA match's Nelson family in Scotland! Isabella had sisters Agnes, Elizabeth, Marion and Jean, and a brother Noben. The images are not available for the Scotland Census, so I can't check that Noben name.
Jean's birth record lists her mother as Marion Armour. This makes the Armour male listed in the household make sense. Marion's birth record lists her parents as David Armour and Marion Riddell.
Isabella's sister Elizabeth married John Beveridge and named one of their son's William Slack Beveridge. William Slack Beveridge was born in 1864, with Isabella having married William Slack in 1859, so it is possible he was named after him.
Edward's birth record lists his parents as Alexander Nelson and Elizabeth Smith.
None of all of the above proves conclusively that Isabella, daughter of Edward Nelson and Marion Armour is MY Isabella, who married William Slack.
To help prove this, I can try to identify other matches in that segment of chromosome 2, from about 130M to 182M.