Without going into the halachic reasons, and very, very basically – Firstly, maariv can only be davened after plag hamincha. Secondly, mincha and maariv have to be davened in two different ‘time zones’. A time zone here means, either of the opinions of plag hamincha or shkiah. So mincha before plag and maariv afterwards is the earliest time one may daven mincha maariv together. You can daven maariv early,but not in the same ‘time zone’ as you daven mincha. Of course, if you daven maariv before nightfall you have to say shma again after nightfall. Also, maariv before nacht is only permissible with a minyan.
LuL, that is a huge question. ‘They’ do anything and everything, depending on who ‘they’ are.