Regarding Spotify versus Google Music, you are right. I just wasn't aware that Spotify puts limits on how many songs you can pin to your device. That alone would stop me from using their service. Also Spotify doesn't allow you to play specific songs on mobile unless you have premium. So in that regard it's just like Play Music.
First of all, I think we both agree that Spotify is clearly not for you, there's really no argument against that.
Now that that's out of the way, you kept repeating that you're surprised about the Spotify limits on 'pinned' music, but you seemed to miss the obvious and important difference about the two services, that what you are 'pinning' on
Play Music without a subscription are
your own personal music collection, while when people are talking about
Spotify without a subscription, not only they don't allow 'pinning' at all, what they would have pinned (if such thing is allowed)
wouldn't be their own personal collection, but instead Spotify's vast library of millions.
My point is, while Spotify and Play Music overall seem to be offering a similar service (and in many aspects, they are), in the particular way
you are needing the service, these two are completely different services that are not even comparable.
An example of other alternatives are Rdio and Pandora, and heck, even iTunes Radio.
For what you need from a music service, Spotify is more like them than Play Music. These are all good services, but I wouldn't suggest any of them to you, because they're clearly not what you need.