Can “sole” really not have a plural referent? For me, number 5 does not sound wrong. And it’s possible to find published examples that support this. Here are two from Google Books for “sole descendants”:
H. G. Wells “The Time Machine” p. 49:
But, gradually, the truth dawned on me: that Man had not remained one species, but had differentiated into two distinct animals: that my graceful children of the Upper-world were not the sole descendants of our generation, but that this bleached, obscene, nocturnal Thing, which had flashed before me, was also heir to all the ages.
And also Charlotte Bronte, “Jane Eyre”:
I am obscure: Rivers is an old name; but of the three sole descendants of the race, two earn the dependent’s crust among strangers, and the third considers himself an alien from his native country – not only for life, but in death.
There are plenty of more recent examples of this as well.