Have you tried using the LP module? In Xposed. I don't know how Xposed nor LP works right now, since my old phone broke and my new phone can't even root and test both Xposed and LP. Also, just asking.. are you trying to install that unsigned apk over an original apk?
The only thing I can think of is, the apk you downloaded from somewhere on the internet is probably corrupted. That happened to me a bunch of times to be honest. Rooted device with LP already applied those 3 patches, couldn't install unsigned nor signed apk(cuz the apk was corrupted during download)