Besides a marriage record, you could also prove Nancy Abbott's marriage to a Westbrook by checking for an estate record or will for Reuben Abbott in Probate Court Records. It might mention her name as Nancy Westbrook and might also mention her husband's name.
Also, check Deed Record Books in Schuyler Co. (or LDS microfilm) to find out what happened to Reuben's property after his death.Look in both the Direct (Grantor) and Reverse or Indirect (Grantee) Indexes for both Abbott & Westbrook, but especially anything that is indexed as Abbott, ___et al. Sometimes I have even found a quit claim which lists all children (and spouses of daughters) including the one that is inheriting the property. Also, look at the direct index for deeds for your Westbrooks and see if any Abbotts signed as witnesses when the Westbrooks sold their property (Grantor).
Unfortunately, the last time I was in Schuyler Co., Mo, I was focussed in on the McVey side of my husband's family and did not research much on Abbotts due to lack of time.
Let me know what you find; maybe we could work out a joint effort using LDS microfilm.