Does Mormon 9 echo the long ending of Mark?

Both give a preaching commission, connect belief and baptism with salvation, and list the same five signs in the same order. Even the unusual detail about surviving a deadly drink appears in the same place.

You identify the specific overlap that an explanation needs to account for.

There is no meaningful parallel because Mormon 9 does not name Mark.

A passage need not name its source to share wording. Compare the phrases themselves before weighing what explains them.

The two codices show that some early copies stopped at verse 8. Irenaeus shows that at least verse 19 circulated under Mark’s name by about 180. An addition can circulate early, so both observations can be true.

You separate when a verse was already in circulation from whether it belonged to the original text.

Irenaeus proves Mark wrote all twenty verses, so the manuscript endings at verse 8 can be ignored.

Irenaeus is early evidence for the longer ending’s circulation, not conclusive proof of Markan authorship. The codices remain relevant witnesses.

A modern writer could use the KJV to compose the speech. An English translation could also use KJV wording for a similar commission Christ gave to Nephite disciples. I’d ask how each explanation accounts for the same sequence of signs and what evidence would distinguish them.

You take the match seriously and keep the English wording separate from the origin of the underlying teaching.

Because I believe Joseph translated ancient records, the KJV wording has no evidentiary value.

That assumes the disputed conclusion. The close English match is relevant evidence and needs an explanation, even if one accepts an ancient-source model.