Is the da necessary in this sentence? da loĝantoj
Yes, kiom is an adverb, and so can't modify a noun on its own. So it's paired with "da," the "little preposition of measure." It's a bit like the pair "how much."
Or like how many?
I interpreted this as, "For example, did they come by way of the Bering strait? Or by car?"
What is the difference between alvenis and venis?