Alsa has had dmix for quite some time. The thing was that until something like a decade ago it had to be manually enabled via a .asoundrc edit. These days Alsa will enable it automatically on any device known to not have a hardware mixer.
Maybe so, but I did try to fix it without resorting to a hardware sound card and couldn't at the time. IIRC the syntax of the ALSA configuration files was particularly arcane.
Not sure why that deserves downvoting.
ALSA refused to do mixing in software for some time. Maybe it still doesn't do it, I don't know. So if you had a hardware sound card that would do mixing, you could play multiple channels with ALSA, but if you didn't, you were SOL.