Hi,
3. In the "S3 location" field, you enter the bucket name (and any sub-path). (I think it says this if you hover over it - is that not happening for you?)
1. It sounds, from what you say, that the S3 endpoint can be either storage.googleapis.com or <mybucket>.storage.googleapis.com
2. You'd need to ask Google what your API keys are, and where to find them. I've not got a Google Cloud Storage account, so don't personally know where they are shown to you. Though, using Google (the search engine!), suggests this:
To generate a developer key:
Go to the Google Developers Console.
Click the name of the project for which you would like to generate a developer key.
In the left sidebar, go to Storage > Cloud Storage > Storage access.
Select Interoperability.
If you have not set up interoperability before, click Enable interoperability access.
Click Create a new key.
From: https://cloud.google.com/storage/docs/migrating
Best wishes,
David