As far as I am aware, you can report a problem, but it will be verified before you are placed on the safebrowsing list.
Most users won't need to because there aren't any issues with their site. It's certainly not a requirement to use Let's Encrypt.
There probably isn't a formal contract. Google has a public SafeBrowsing API that can be used.
Let's Encrypt doesn't want to issue certificates for hacked and marked phishing domains. At the least, they have a bit of a need to ensure some level of checking. There aren't a lot of ways to do this. You could use Google's SafeBrowsing data or a similar service, like the one that is offered by Yandex. SafeBrowsing is generally considered high quality and quick, so it's a good fit. The only other method is to have real humans review each request, which eliminates the focus of Let's Encrypt on offering automated and free certificates. Alternately, you could just not offer any kind of quality control, which would likely get you removed from browser trust stores quickly, killing the service.
If it's wrongful, then it's an easy fix. See the details of why Google has you on the SafeBrowsing list and protest that.
How do you know without looking at the details for why you're on the list?
Either way, it looks to me like the domain isn't on the list anymore, using the public Site Status tool. Maybe you can try to make the request again?
Also, if you're that unhappy with Let's Encrypt, there are several paid providers you could turn to for a certificate.