Are Pokemon Go eggs predetermined?

Yes, the Pokemons which are going to spawn from the egg are already predetermined as soon as you obtain the egg. And you’ve yourself provided an evidence to support this – the Pokemon ID associated with the Egg.

How are egg stats determined?

While it is not possible to know what will hatch from an Egg beforehand, an Egg’s properties are determined at the time it is obtained (not when it is hatched). The hatched Pokémon’s Power Up level will match the player’s Trainer level at the time its Egg was obtained, capped at level 20.