'-In what circumstances would it be normal for Henry to live with is master at 15, rather than his family? (Could it be that Susan deceased and Robert in the Workhouse)?'
'Children' younger than 15 frequently found living in other households, particularly as apprentices.
'-Why can I find barely any trace of Robert and/or Susannah - My best assumption is that there were different names used or that they were born abroad?'
Lots of reasons why you cannot find them, including records damaged/destroyed, or births/marriages/deaths not recorded, or records not yet transcribed.
Rather a large jump to assume they were abroad.
Where was Henry, 1826, born, and have you found any potential siblings for him?