The passage below is from the website as follows:

The new African form, with its fancy tools, ochre paint and shell-bead ornaments, might have displaced its neighbours, but it would now settle down to enjoy its million years in the sun before gracefully giving way to something new. This time, however, some of the L3 people promptly spilled out of Africa and exploded into global dominion.

In this text I have a two question.
First, it seems to me there is a disagreement of tense between ‘might have displaced’ and ‘would settle.’ For me, it seems better to change the latter into ‘would have settled’ to agree with the former. What do you think of my line of thought?

Anyhow, regardless of whether the question is right or wrong, why did the author make such a ‘gap’ in the sense of tense?

Second, is it all right if the conjunction ‘but’ be replaced by ‘instead’?

Thanks in advance.