I have a partial answer to one of your questions I think.
There is no requirement that evolving into a new species needs to cause extinction of the ancestral population.
That is, just because humans evolved, does not mean anything humans evolved from had to all perish. Each species benefits from their own fitness, allowing them to contribute many offspring to the next generation.
Arguably, the previous species was successful enough to where something new could emerge from and so the remaining populations continued to be successful.
Now they may have high fitness but they might not do adaption well. That’s where they acquire traits suited to the environment over many generations and in some cases if there were extreme changes whereby the evolved species had protections but those they came from did not, then one might see the species start to die out.[responsivevoice_button voice="US English Male"]