a) Discuss mids/maps and how you want to play them?
At first, we kinda just freestyle mids and see what works and what we are comfortable with. We adjust as needed and try to find a few good working strategies. A common misconception is that invite teams have ridiculously complex strats, but that's not really true all of the time. It's just that invite teams are much better at executing all strats than lesser-skilled teams, no matter how basic the strat is.
b) Talk about your playstyle and how you want to refine it as the season goes on?
This is only necessary when someone doesn't do enough of what they should be doing to help the team. Like if a roamer isn't bombing enough, or if people die at times where deaths aren't necessary.
c) Do your teams actually go over demos of yourself (or your team)/other teams to refine your style and how you react to other teams?
It's never a bad idea to do this, but I think any team I've been on would do it if we were trying to figure out what's wrong with our mids.
I mean in the past, I even remember hearing about teams during the EG times were actually going over demos in their hotel room just so they could win that season (s9?), is this still the case?
Eh, it depends. It definitely helped blight-own during s6 finals against my team, because they watched our granary demos and countered our mids incredibly (and granary is more dependent on mids than any map). We got rolled in like 10 minutes, and we were very strong on granary, too. If they didn't study our stuff, it surely would have been much closer. Aside from that, some teams still might do things similar, but it's not ridiculously common as you'd think.