Einstein would. He's the one who came up with the concept of space-time, IIRC.
Yes, but also Einstein came up with general relativity, and the concept that values are relational.
Einstein wouldn't be pleased with a REAL fixed grid. It would mean that everything's not relative each to the other thing, but relative only to the grid.
Of course, if there really is a fixed grid, it would explain why the frenetic search for the "gravitron", which makes no sense in a truly relativistic Einsteinian universe that has no ether, but makes sense if it's the thing that grabs onto the fixed grid and "warps" it.