So what’s the best explanation for Lemnian being related to Etruscan?

The cautious answer is that Lemnian, Etruscan, and Rhaetic show enough similarities to be grouped as a language family, often called Tyrsenian. That points to a historical connection, but it doesn’t tell us whether people moved from one region to another, when they did so, or which direction they traveled.

The inscriptions are a small sample, so the relationship is clearer than the full story behind it. A shared language family is evidence of connection, not a map of migration.

I’d separate two questions: what language the inscriptions use, and why speakers of it were on Lemnos. The first has a reasonable answer. The second is much harder. A local community with ties to other parts of the Aegean is one possibility, as is movement of people across the region, but the inscriptions alone can’t choose between those explanations.

Calling the stone a memorial for one visiting speaker doesn’t explain the other Lemnian evidence mentioned here, though that still doesn’t prove a single, large-scale migration.

There’s a basic numbers problem. A few inscriptions can help establish that a language existed and reveal some patterns, but they don’t give researchers the kind of material needed to translate everything or reconstruct its history in detail. So claims about a precise homeland or a dramatic journey are a lot more certain-sounding than the evidence allows.

The intriguing part is real enough. Lemnian appears connected to languages found much farther west. The route and timing remain open questions.