Hi Lydart,
realise this is a very old thread, but, as a memorial mason, I'd like to put my tuppence worth in...
The Palm trees are a Christian symbol...Palm Sunday...
On your third photo at the start of the thread there is a figure, normally The Good Shepherd (Jesus) kneeling under the Palm Trees in front of a broken column. This is broken which indicates the death of a young person ie one who did not reach 3 score years and 10. Each Stonemason would have a section of designs which they favoured, all variations on a theme.
As far as Masons marks, it would be extremely unusual for one to be anywhere on a family memorial. A building or public monument yes, and even then only near the bottom.
As an aside, I've swam in the water under the Britannia bridge when doing a Rescue Boat course with the Scottish Fire & Rescue Service....great fun!