A native speaker commended someone for investigating something thoroughly, so they said "for your relentless investigating efforts", then they corrected themselves and said "unrelentless", then finally they thought maybe "relentless" was right after all. I suggested "unrelenting".

Wiktionary states this use of "unrelentless" might be a malapropism. The non-standard variant appears in Ngrams, there has been continued interest for the word (trends), the possible solecism is discussed in the context of other words beyond just discarding it, some opinions from other forums date to almost 20 years back so there is value in seeing if these have changed, and it also appears in some song lyrics (like Alanis Morissette's Big Bad Love (1992) "I wonder why I am so unrelentless / There's nothing that I can do to prevent this...") which seemingly haven't been corrected in the last 30 years.

Do you accept, or is "unrelentless" generally accepted to mean, "relentless" in this context; is it a malapropism or not, a solecism or a non-standard variant, and does that vary with regions? Which of the three adjectives do you find better suited in this context, and why?