Not uncommon for convicts to change their name to get away from their criminal past. He may have wanted to disguise his Jewish name for other reasons too.
I could see no records of a Samuel Cohen in Tasmania after the date of his pardon. Coming from London, he would have moved quite happily to Sydney.
I doubt he was a very skilled baker by the age of 18. He may have been working for a baker.
I am having trouble reading all the writing on the various records, but on one sheet there is a mention of a Daniel Lipman in Newington (London), also Philip - baker. I think Samuel's father was named Moses.
Edited.