I think 'lied about everything' sounds a bit drastic! If they didn't actually marry, for whatever reason, all the lies that come after that would be pretty normal ones!
I'm not sure why you would think they would have had a marriage licence if William was under age? They wouldn't have needed one in order to marry - in normal circumstances the vicar would just have needed the word of a parent for consent.
Now, Marshall Row - I looked to see if there were any clues in this street. In 1841 and 1851 we have the same couple living there:
1841
James Martin 60 Ag Lab Yes
Mary Martin 55 Yes
Sarah Martin 21 Yes
1851
James Martin head married 68 labourer b Wivelsfield, Sussex
Mary Martin wife married 73 b ditto
I'd say there's some chance the William Martin who died in 1849 aged 29 is connected with this couple and may be their son (but not necessarily your William!). There's no obvious bap for Sarah (presuming she is their daughter, which of course she may not be), but we don't know where in Sussex she was born.
EDIT: The family next door to James and Mary in 1851 are William and Sarah Skinner (aged 30 b Brighton). I wondered if this was the Sarah Martin who appears with James and Mary in 1841. I looked briefly for a marriage 1841-1851 on FreeBMD and found nothing, but when I checked out the registration of their dau, Emily aged 1 in 1851 ....surprise, surprise!
SKINNER, EMILY mmn MARTIN
GRO Reference: 1849 S Quarter in BRIGHTHELMSTON Volume 07 Page 309
Looking further, they married in Whitechapel in London:
https://www.ancestry.co.uk/interacti...nSearchResults
Sarah says her father is James, labourer.
I think the Skinner's may have had two children before their marriage (not Betsy - after her) both registered as Skinner with mmn Martin, but more research would be needed to be more certain there isn't another couple with the same surnames. Both those children seem to have died by 1851. I can't be sure that Betsy (or maybe Elizabeth/Eliza etc) was registered at all (assuming she was registered as a Martin, but we don't know what name she might have started life with!).
The next issue is that I can't find an obvious bap for Sarah in or near Brighton with father James (I didn't include mother's forename as we don't know when James married Mary). Of course I also can't find a bap for James and Mary's son (if that's who William who died in 1849 was) either!
None of the above is of much consequence if it's not the right William, but I suppose gradually untangling different Williams might help in the long run!