Swan says that the verb is singular because the subject is one. ( Practical English Usage, 1984 edition, entry 442, one of...)
If he's talking about a sentence with the same structure as the the one in the OP, he's wrong.
"Harris is one of those who
are certain to be in the soccer coach first XI".
The prepositional phrase is "of those". And the relative clause, "who are certain to be in the soccer coach first XI" refers to the plural determiner, "those", not to the subject, "one" or "Harris".
There are those who
are certain to be in the soccer coach first XI, and Harris is on of them.
Of those who
are certain to be in the soccer coach first XI, Harris is one.
What is he one of? Certainly not "those who is certain ..."
The number of the subject is irrelevant.
"Harris is one of those who are .."
"Harris and Smith are two of those who are ..."
There is a context in which "is" is correct. And that is where not all of "those" are "certain to be in it."
A: "See those 100 people over there? Harry is one of those who
is certain to be in the coaching."
B: "How many do you think are certain to be in it?
A: About 10. Harry is one of those [10] who
are certain to be in it. Only about one in 10 of those people
is certain to be in it. Harry is one of those who
are.
I find that I disagree with Swan on a number of issues, and I find this disturbing, because it seems that one of us must be wrong.