Actually I am not worried at all about the dovetail coming out. Pine does not shrink that much! Plus there will be minimal horizontal thrust on the plate itself as the rafters will terminate into the tie beam. It is the tie beam itself that would have to absorb that horizontal thrust.

Also not worried about shear on the top plate. remaining wood would be an effective 7x8 and for my area would feel this to be sufficient. I am worried about shear on the tie beam itself at the dovetail though and will go back to them and discuss options I guess. It if funny how people get something in their mind and 'have to have it' like this. Also not so keen on draftspeople drawing up a frame with no knowledge on joinery!

We actually started with a 12:12 pitch roof and had to go down to 6:12 due to some windows on the second floor that would have been partially covered by the 12:12 roof.