Hi, I have a small amount info as I have not concentrated much on the Dewar name but I really hope it can help. Here we go:

I am thinking your Dewar family and mine could be related.
My gg grandmother is Catherine McCuaig McNamara, her mother is Sara Dewar (Dewar is her maiden name.) Donald McCuaig is her husband.

In 2008 in conversation with another ancestry member (not confirmed by me.) I was told that Sara and Donald were Married in March 1824 in Lochiel Twp Glengary ON.

What I also found interesting is that the McCuaig family is from Glenelg, Scotland and departed on the Baltic Merchant in 1815 this info I found myself online on "theclearances.org"

As far as I know Sara and Donald stayed in Canada.
Catherine and her husband and children moved to the states.

My family tree is on Ancestry under my grandfather: Reginald McNamara (if you think it maybe helpful)

I hope there was something I gave you that could break that darn brick wall!

Best of luck.