“Just to put things straight”: This is your view, conjectural rather than factual

“James Norris Hall Forbes Stuart and Norris Hall Forbes were 2 different people.” You’ve produced no evidence to support this view.
“Although both were born in Woburn, Bedfordshire, England,” - there’s only evidence of one birth
“ I have James N.H.F. Stuart's marriage certificate and his father was James Stuart”. You have the marriage certificate for someone calling himself James Stuart (not James N.H.F Stuart) who names his father as James Stuart. No evidence would have been required and he could have called himself any name he chose. (Numerous illegitimate people invented a father’s name on their marriage certificate to hide their illegitimacy).
“This is in line with family information for James NHF Stuart.” Yet you say “apparently James NHF Stuart wouldn't speak about his family,”

It’s your prerogative to believe that Norris Hall Forbes and James Norris Hall Forbes Stuart were two separate people. But you can’t prove they were. More than one researcher on these boards believes they were the same person. We can’t prove that either! It boils down to circumstantial evidence and the balance of probabilities.

I understand that the only occasions when he was recorded as James Norris Hall Forbes Stuart were the 1911 census, when the schedule was completed and signed by his son in law, and his death registration, also obviously completed by someone else. In all other records from his marriage onwards he is plain James Stuart. So far as we know he never used James Norris Hall Forbes Stuart himself. An alternative explanation for the addition of middle names could be that in 1911 his son in law was giving a hint to the world as to the real identity of James Stuart – that he was actually Norris Hall Forbes – or that the son in law had heard him referred to as Norris Hall Forbes and assumed these were middle names.

As it stands, there are records up to 1857 of Norris Hall Forbes – baptism, censuses, school, army, but he then disappears. Rising like a phoenix in 1872 was James Stuart, of whom there is no trace before this date. Had it not been for his son in law completing the 1911 census in the way he did it’s possible that we might never have linked the two names.

What are the odds of there being two people born in Woburn at the same time, one Norris Hall Forbes who is well documented, and the other, James Norris Hall Forbes Stuart of whom there is no trace until 1911?

Unless you find a birth or baptism of James Stuart you’re not going to convince me that they’re two separate people, and similarly I doubt that we’ll find any document that says something like “I, James Stuart, alias Norris Hall Forbes…” which is what it appears to need to convince you that they are the same person.

I think we must agree to disagree!

David