It's not "my" take, it's the SC's take. The link posted by Bob makes this clear. And no, they didn't say the votes couldn't be counted, they said they had to be separated. Which, by the way, PA was already doing.
The ballots in question could potentially be thrown out depending on how the court challenge plays out.
www.forbes.com
Who certifies the vote in each state? It's different by state. In Michigan it's a state board of canvassers (not the legislature)
www.michigan.gov
In Georgia it's the Secretary of State and the Governor (not the legislature)
Republican Georgia Gov. Brian Kemp on Friday signed the paperwork that officially grants the state's 16 electoral votes to President-elect Joe Biden.
www.cnn.com
In Pennsylvania it's the Secretary of State (not the legislature)
Commitment 2020
www.wgal.com
These are the laws in those states. When a state legislature has delegated power to other entities to conduct elections, those entities can change rules as necessary. I don't know why you mentioned mayors, at no point has a mayor been involved in this. Changing rules and writing laws are not the same thing
And, in PA, it WAS the (Republican-controlled) legislature that changed the laws to allow mail voting, and it was used in a previous election, too. No one seemed to throw a fit then. See my post above for the link.
I'm sorry, but you're just wrong. If it were actually illegal for these rules to have been changed by someone other than the legislature, then all of these lawsuits would've been slam dunks. And, yet, case after case has been dropped or dismissed.