Although they are probably all connected at some point....Records reflect you are correct, they were not brothers but cousins of some sort. Infact, Floyd & Donald didnt even have the same parents.
The Floyd Swails that you are taking about has a nice OBIT on Findagrave.com
From his year of birth & location.......on the census records he is the s/o Forest Edwin Swails & Josie Jackson.... This Forest is 'not' the Forest (bn 1909) brother to your William.
Forest Edwin Swails bn 25 Jun 1899 Boone Co, IN died: 26 Nov 1991 md25 Dec 1918 in Boone Co, IN Josie Jackson This Forest Swails appears to be the s/o: William Swails bn abt 1862 & Ellen bn abt 1861
And the only Donald Swails I found that died in Carmel (1982) is listed on the Indiana Birth records as: Donald H Swails, jr Bn: 17 Aug 1917 s/o Donald Henry Swails & Katherine Pedigo Donald Swails (jr) is refered to Chief Swails regarding a memorial fund..........so probably the same one.
Have you ordered a copy of his original Social Security application???He would have had to produce a birth certificate or a signed statement as to his birth to obtain a SS number.Not always, but most often they put the real parents names on the application because they wanted it to match the other paperwork they turned in. Sure worth a try.
Check with the Cemetery where your William has records.Try to get OBITS or death certificates for all those you believe are the siblings of your William.Never know where the next clue will come from.