"Alt-Left" isn't a term. The only reason "Alt-Right" is a term is because that's what the right wing groups that hated the GOP establishment started calling themselves.
For the left it's usually just "progressives"
Mmm, no. "Alt-left" was an established term before Trump came along, used to refer to the subsets of the left that peddled "anti-war rhetoric" that was literally just war crimes denial and support for dictators, coined by those on the left who DIDN'T ascribe to such conspiracy theories. The right tried to co-opt the term to hit the entirety of the left over the head with, but that doesn't mean that there isn't an actually existing "alt-left".
As for what these people will do? They'll continue to peddle racist conspiracy theories about brown people protesting for democracy being a CIA psyop, like they have for decades already. Unfortunately, these people aren't going away.
Not true. Alt Left just started up as a counter to people who thought "alt right" was some kind of insult left wing people made up, even though it never was. No one ever referred to themselves as "alt left" because the left wing in this country was always marginalized anyway except in the last 4-6 years maybe.
Hence there couldn't be an "alt left" because there really wasn't any "mainstream" or "establishment" left.