Yes, you should key The Younger as a suffix even though it is not in the dropdown list. Sir is definitely to be keyed as a prefix. As far as I know, Baronet should be keyed as a suffix on this project unless WAP say not to.
Are you sure it doesn't say Countess Mexborough, rather than Wexborough? I get a lot of Google matches for Countess Mexborough. The Right Honourable would be keyed as a prefix and I think Countess Mexborough would be the suffix (Mexborough or Wexborough isn't her surname.)
You are right about widow and spinster, they are not keyed as suffixes.
And you are right about how to key a name that is split over two images.