Is it "safe" to provide users with the apk file?
There is nothing dangerous about it, and this is done occasionally when the store isn't accessible.

In response to your second question, apks are not unique - and can be easily transferred to another device, or the internet. This is the base of android app piracy.

When you upload the APK to Google Play, GPlay does nothing to it. It is up to the developer to obfuscate the code, to prevent decompilation.