Iam a little bit confused by what this sentence means. (highlighted one in blue)

It basically means that he didn't want to be distracted from his plans for their gain.

He did not allow himself to be diverted

diverted= changed, in another direction

from his schemes

schemes= ideas, plans

by considerations

considerations= something thought about, a decision that you don't have to take but may want to. 

of mercantile gain.

mercantile= things, possessions, money

He didn't want himself to think differently about his plans for a decision that may lead him to gain possessions, things, etc. 

I hope this helps!
~kaikers

The details that best show evidence of tribal traditions are, "never in the history of his people had a man married a woman who spoke a different tongue” and "it has never been heard," was the verdict of an old man speaking a few weeks later.