Hi Morney
It does sound confusing

You say you have a marriage cert for him in Newcastle in 1909? However, wife Annie didn't die until 1918

What showed for Hugh on his wife's death cert: employment, etc. What marital status showed for Annie?
This is Hugh and Annie in 1901:
Hugh Levitt 29, Foreman (steam Boiler), b. British Subject, Rawal Bridee India
Annie Levitt 28, b. Ireland
Mary Levitt 8, b. Falkirk
Rosina Levitt 4, b. Falkirk
John Levitt 2, b. Falkirk
Address: 88 Cochrane St, Bathgate
I would say Hugh above is certainly v. likely to be the same Hugh we have in 1891 with his parents Hugh and Marion (McIver). I wouldn't worry about the spellings of his surname. I would imagine you will see a number of spelling variations on this name.
I would imagine his father Hugh was probably in the Army given the varied birth places for the two sons in the 1891 entry. He would probably have retired from the Army by 1891 where he shows as Labourer, retired.
Where and when did Hugh die (you said the Poorhouse)? Have you tried checking to see whether you could trace any Poor Relief applications?
We like a good mystery here on RootsChat

Monica