Lesley.
I'm afraid the Michael MIs aren't going to help.
I check the Michael burials and only found 2 Margaret Garretts I can't account for
Margaret Garrett bur 30 Aug 1788
Margaret Garrat bur 16 Mar 1802

Neither has any other identifying info.

You will probably need to check the will os the mother Isabel McBooy which is in Braddan 1813
I see in the Braddan Burial Register that Isabel's body was sent to Michael for burial but she doesn't appear in Michael Register.
None of the people you mentioned have MIs in Braddan.

I did note a David Cowley 75 buried 13 Dec 1836 Braddan
David is fairly unique
There are a bunch of Margaret Cowleys in Braddan
I found no suitable Marg Cowley MI in Braddan and I found none for David.

DJD