I assume path will be: World of Warcraft/_classic_/Wow.exe instead of World of Warcraft/_retail_/Wow.exe
Although I am not sure, but I think beta/stress test was in a path similar to that so feels like they would put classic there.

In some expansion wow added a hidden window that messed up early HKN scripts, I think it will have the hidden window in classic so old scripts might mess up with launching and renaming, but I think that was added in cataclysm.

Some have said it works to use 1 classic client for each active wow account on your battlenet account, so that should work fine. I assume the login page will work exactly the same as retail, but I haven't tried beta or stress test.

At the 13th they will start the login servers so we can make characters but not play, so then we can make sure we can properly launch it. Plenty of time to fix any issues, if there is any, before classic properly launch.

Ingame it should work fine, but ofcourse you have to rewrite your script to properly handle your new classic team (maybe add new round robin buttons, change hotkeys etc).