I think the tin was a very flimsy excuse and a reason to go there on a non visit day. I wonder if there actually is a tin, has that been proven ? has MM ( grandmother ) confirmed that she was given a tin when she arrived back at CH with AG ?
Also, if there had been a birthday cake, which had been given to SH the week before ( on her birthday ) then surely the most likely place for this to happen would have been CH.
And, wouldn't it be normal to cut the cake there and then and hand round slices.
So I suppose the only way SH ended up with the tin would be if they didn't finish the cake and she took the remainder of it back with her to CML, using the tin to transport it.
I'd like to have heard from MM about the cake tin, e.g. did she want it back quickly, was it rare for MM to go to CH and so it was important the cake tin go back that day.
I wouldn't like to be on a jury, I'd want to be asking my own questions.