Sorry my sentence was ambiguous. The “it” that doesn’t support Android 7 is the chipset and not Google.
Let’s not get into that discussion again. If you’re interested it has been discussed e.g. here, here and here.
In short: As you say the Lineage port proves that the chipset can handle Android 7 very well, but Google decides that “the chipset doesn’t support Android 7” for whatever selfish reasons - so no phone with that chipset will ever have official Google-supported Android 7.
Well I’m sure the wearefairphone community developers would appreciate donations. 
Well you can’t switch between FP OS, Open OS and Lineage OS without wiping, but upgrading within one OS line works without wiping data (obviously the Lineage OS port didn’t experience an upgrade yet, but I don’t think it will require wiping if you upgrade from Android 7 to 8).