Is the "Remote Application Removal Feature" also effective for apps from samsung and other alternative stores?

Is [Remote Application Removal Feature] also effective for apps downloaded from alternative stores?

No. Google doesn't care about apps that are downloaded from alternative stores, nor does they have to. Google provides a chance for user to check the application on installation-time (called Package Verifier), but beyond that, it's not Google's responsibility anymore.

If so, why not make use of this feature to remove repackaged apps downloaded from, e.g., stores of China.

I'm not sure about the connection between malicious apps and repackaged apps. The removal feature is for malicious apps, while repackaged apps are more about copyright issue, which is outside of Google's responsibility.

If I report to Google that one of my apps has been repackaged and the repackaged app is on some alternative stores, does Google have the privileges to take it down?

Again, no, it's not Google's responsibility. This is a legal issue regarding copyright between the developer and that alternative store, not Google.

Does Google use this feature a lot to protect IP of legitimate apps? Are there any reports or documents about such statics?

I'm not sure about that, but regarding IP protection, Google might have blocked them before it's even published on Play Store (as per Play Store Developer Policy).