Many thanks for your efforts on this. You have got further than me, particularly on finding Graizelound (very convincing) and checking out the signatures.
Putting the wedding licence to one side, the dates all look coherent. In particular, Richard's death in 1762 fits well with his brother writing a will in 1763 in which wasn't mentioned, and as you point out even if the Elizabeth who died in 1764 was the sister, the date is not inconsistent (although a burial in Haxey would have been neater).
The marriage licence is annoying though. The age of 28 is very specific, when 'over 21' would have done. And a baptism for Elizabeth would have been nice.
I have been trying and failing to find other wills that might shed extra light - Williams, Todd, Yates or Saunderson. I still hope to get a copy of the Todd/Tong marriage licence, but I don't see how this will help with the Tongs.
In the absence of any other information, I think that your suggestion is best, despite the inconsistency over the dob.
Thanks again for all your insights and hard work.
Alan
PS I also wonder about the late brother in law Griffith Williams, why he had two clerks but was unable to make sufficient provision for them himself, but have drawn a blank on this too.