Outstanding support issues, inconsistent support reps

I am having major ongoing issues with this plugin. I have multiple support tickets open but the support I am getting is inconsistent – I often get responses from BC support who are not familiar with this plugin. I then ask clarifying questions and a few days later will get a response from another person. This has gone on for months. I am reaching out here in the hopes that someone like @topher1kenobe can look into my issues and we can get a dialogue going, because talking to a different person every few days is never going to solve these problems. I cannot continue to have a site that’s broken, while waiting a few days every time I send a response, only to have it answered by someone who is not familiar with the software.

Here is a very general overview of my issues:
I am continuing to get messages almost daily from WP saying that the site has had a technical issue. Sometimes “There has been a critical error on your website. Learn more about debugging in WordPress.” appears on the front end of the site. This appears to be related to API credentials, but my client is the store owner, because he needs to have access to the financial data; this means that I, as the site developer, do not have access to the API credentials because I’m not the store owner. I also reported back in March that checkout doesn’t work in Safari, IE11, and occasionally Chrome. This issue has sat unresolved since then.