Sorry I didn't see this before. Thank you for your reply.
1. In my few tests, 14 users sent data within around 10 seconds and only 10 or 12 made it into the spreadsheet.
2. No. I'm not familiar with LockService. I'm actually not a programmer. I just was able to get a couple pieces of scripts here and there, and tweaked them for my website to work. But I just Googled LockService and it looks like this can help. Does LockService need to be in each function block or just one per entire script?
3. The article you sent answers my question about the Apps Script limitations. Looks like 60 is the top number. That's fine with me. I will read more about LockService, it looks like that's what I need right now.