Has Christopher Tolkien ever publicly spoken about George R. R. Martin or his work?
The present Perfect is often used with "ever" to ask about an experience in the past, for example
- Have you ever seen the Sistine Chapel?
That person may not have visited the Vatican City or have seen the Sistine Chapel but it does not exclude the possibility of their experiencing that event at some point in the future.
- Has your child ever eaten shellfish?
Likewise someone may never have eaten shellfish in their life but this situation could change next week, next month or next year.
The Present Perfect construction is not usually used with people who are dead. It is impossible for Christopher Tolkien, a person who died in January 2020, to presently speak about Martin. The act of speaking cannot be performed or repeated in the present nor in the future.
It would therefore be more natural to ask
Did Christopher Tolkien ever speak publicly about George R.R. Martin?
The act is confined to a specific moment in the past.
The original construction using the Past Perfect is also correct; albeit it suggests that before his death Christopher Tolkien had discussed works of other writers in public, not only his father's, which is totally plausible.
[Before his death in 2020] Had C.Tolkien ever spoken publicly about George R.R. Martin