The answer to 1.2 is both. Threema is encrypted, so G00gle can’t see the messages. But contacts get uploaded.
I think you also have a third option: Installing microG. MicroG makes apps think that GMS are installed, so you don’t get the annoying “This app doesn’t work without G00lge Services” and are able to use most apps normally.
See Level 5 Step 5 here on how to install microG.