1) It's not a fork. There's no such thing as an Amazon specific fork of the Elasticsearch git repo in the opendistro github account. There are no Amazon specific patches to Elasticsearch.
2) Instead, Amazon redistributes a vanilla OSS build of Elastic. As is and unmodified. Producing these builds is and always has been a feature of the elasticsearch build scripts. With every release they produce OSS binaries and OSS docker containers (both without the closed source plugins) in addition to the ones that include their x-pack components. All Amazon does is take those builds and bundle their own OSS plugins.
3) What is and is not open source is clearly documented in the Elastic repository. There is zero ambiguity here (legal or otherwise) unlike what Amazon implies in their marketing. If it's in the x-pack directory, it may be closed sourced (some plugins are OSS). If that bothers you, use the before mentioned OSS builds. Everything outside the x-pack directory is OSS. OSS here qualifies as Apache 2.0 licensed or compatible. It's that simple.
The OSS plugins that amazon provides are of course nice if you need them. Less nice is that they seem to be perpetually several releases behind Elastic with both the plugins and opendistro. So if you use this, you are running with known & fixed bugs that may or may not affect you. You could argue that Amazon maybe does a lot of testing. If so, those tests don't appear to be part of their OSS repos. The other explanation is of course that they only update their cloud service a couple of times a year and simply ignore bug-fixes or even patch releases to what they shipped, other fixes and improvements that happen upstream, etc. Or even any documentation for that (refer to the Elastic official release notes and documentation for what was actually fixed). If these bugs happen to affect you, you are on your own. The release notes are "whatever Elastic said a few months ago". Refer to the Amazon release notes here if you don't believe me: https://opendistro.github.io/for-elasticsearch-docs/version-.... The last few releases were basically "bump the version number" and absolutely nothing else that Amazon considered worth reporting.
So, if you are comfortable running that in production use it at your own peril. I'd argue it's probably better to take the latest Elastic oss build, fork the amazon plugins you actually need (if any) and simply bump the version numbers to match the current elastic version. Amazon seems to do little more than that between releases; so you are not really missing out on any meaningful QA, support, or other stuff Amazon implies they are doing that they are clearly not doing.