I started monetizing https://sheetsu.com two months ago. It's profitable right now. I'm the only one involved, wearer of many hats. I'm opening my Baremetrics.com dashboard soon to be transparent.
Yes. Each document is separate API. Paid plans support multiple sheets/tabs. Also, on the paid plan you've got bigger quota of available APIs/Sheets you can create and number of hits you can perform to each API.