Kim,
Don't think I can help with your research but pass on the following that you might find useful (in case you didn't know);
1) Majority of my Hewetson ancestors originated in and around Cumwhitton, which is less than a mile from Cumrew, and though they pop up in various places none have a Cumrew connection as I recall.
2) The spelling of the name is generally as Hewetson, though there are a number of instances where they were recorded as Hewitson. (An example is my gran who was so recorded on the occasion of her baptism at Cumwhitton and marriage at Carlisle).
I did find a Hewetson/Ivinson link at Carew on ancestry.co http://awt.ancestry.co.uk/cgi-bin/igm.cgi?
Though obviously not your Sarah it suggests you are on the right track, with with likely ancestors there.
Good hunting
Jolyon