Ed Townsend's estate only holds a PA
(writer's) copyright on the melody, words and vaguely, the chord changes
on Let's Get It On. While a lot of the changes are similar, they're not exclusive,
and the melodies are totally different. IMO, the suit will fail based on all of that being true.
The rightful party to sue Sheeran, in this case, would be Motown Records, who
holds the SR copyright (a particular & unique configuration of sounds).
on the actual recording, from which it was sampled, if they were used
w/o permission.
Most probably, a fee was paid to obtain those samples from the original record, as
is now the absolute necessity in big time production.
