What is the alternative to get an appendblob from a container?
Check your WindowsAzure.Storage version. As I test, the version>= 5.0.2 of MicrosoftAzure Storage contains a definition for GetAppendBlobReference. You could follow the SDK.
What is the alternative to get an appendblob from a container?
You could use Rest API to get the append blob. Append Block is supported only on version 2015-02-32 version or later.
For more detail, you could read this article.