> And another question: how else should the title on the sign be tagged if NOT as a name?

we have https://wiki.openstreetmap.org/wiki/Key:inscription

> And I simply don't see any problem with tagging a descriptive name as a name, as long as you haven't invented this descriptive name yourself, but there is clear evidence on site that a name was given for something.

So if oak would have sign "Oak" (for example in botanical garden) then you would put name=Oak?