1. Interactive Schedules can't do calculation based on the quantities.
It is a long standing wish for it to do this.
2. You can get a SUM of you quantities (in this case cost) so long as you activate the "SUM" button in the "Fields" section of the schedule settings.
You will need to add a field for the cost parameter and then activate the sum button - similar to the attached image.
3. I can't say about this as I don't use zones in any of my schedules.
But I had a quick play and could not get this to work either.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10