Will try to answer your different questions.
Change Feed support. I don't understand your comment. Change Feed is defined for a container, not for a subset of partition keys. Change feed works the same regardless of partition keys. A logical partition is defined as a combination of one or more partition key values within the array of partition key paths. Logical partition keys remain 20 GB in size as does physical partitions. Change feed operates at the physical partition level so can be parallelized by physical partition.
There are multiple ways users can determine proper use of partition keys within queries. Reviewing Client Diagnostics to look for requests that are fanning out across a large number of partitions, looking at Query Metrics and run queries in the Portal as well as using Dianostic logs. At smaller scale (< 10K RU or 50GB of storage), queries that do not use the partition key will run on the same physical partition. But such queries will not perform at scale which is why it is recommended users understand their partitioning strategy as well as conduct tests at scale.
For the last two questions, I cannot share forward looking dates on when features or various aspects of them will be released or otherwise share nonpublic information.
Thank you for your inquiry.