#1 - I'm not sure how to check these permissions honestly, but the alternative is that your computer isn't working properly. It's most likely the permissions and nothing on your machine.
#2 - You do have this problem. On my sheet I set the reminder days for each customer individually. Most customers are set to 8 days if that's what you're wondering. This is done automatically, you won't have this problem if you don't screw around with it. To fix this problem, we can revert to the original template.
#3 - I totally get where you're coming from. During development I was working out a few different bugs. The labels aren't something I do for you now, it's something you would have have to switch up yourself. If I "fixed" this on my end, your gmail inbox would only appear to get more complicated. Just specify the labels yourself in the script.
I looked into your Google Sheet and I have no idea how to do what you're asking. My sheet was not intended to support multiple languages, or multiple clients with the same Client ID. Make sure you have unique Client IDs and that your dSupportSuite package is emailing you in English. According to your google sheet, you have not received any maintenance or heartbeat emails (ever). Also, I get a permissions prompt every time I run "onInstall" and "onOpen" which I assume you already approved.
I set up a second sheet for you if you want to play around with it.