Hi, I have been using iTunes from the beginning also, and frankly, this is the worst it's been imo.
I have never used iTunesMatch or AppleMusic, as everything worked fine for me.
GooglePlay is not even considered here.
There are two AirportExpress, an AppleTV and an Airport TC taking care of the Wi-Fi (music) distribution around the house, sometimes they play up, but generally it works fine, specially when I can control everything from my iPhone with the RemoteApp.
On that note, did you check if your iPhone is on EXACTLY the same Wi-Fi and AppleID?
This fixed it for me, finally.
When I got home, I decided to give it another try...
Checked the Wi-Fi network iPhone was on, and it seems it had inadvertedly logged on to my "other" wi-fi network.
(All devices must be on the same wi-fi network and using the same AppleID for Remote to work, sometimes after an update, or after restarting, they will log on another previously used network if you have more that one... "e.g. Guest")
Started up iTunes on MBP and could acces the iTunes Library located on the iMac, reinstalled Remote.app on iPhone and proceeded to "add an iTunes Library" through the app's Settings, the four-digit box appeared, and on the MBP's iTunes, the Remote icon button appeared right beneath the play button, where usually the iPhone icon sits when it is connected through USB. Clicked that, filled in the four digits, and hey presto, both my iMac and MBP appeared on the iPhone app, as well as the AppleTV!
I now am able to select and control either Library... just the way I used to.
I would urge you to give it another go, I see your frustration with it, but hey, I solved mine through perseverance!
Maybe you could post screenshots, I believe your solution lies close to the one I found. Don't just give up!