how can the first be killed if it comes back in season 7?

It's not killed. At the end, Cassandra says, The First cannot be killed as "It's an integral part of the universe" but will take centuries to regain it's power to be a threat again etc. Something along those lines anyways.

But yes, the setting of the game seems illogical, (between Season 5's Forever and Intervention.
How did Ethan escape in the first place? Faith is in jail over on Angel and if The First cannot return for hundreds of years or so, how can it return in Season Seven?

(Unless you consider Giles and Anya's convo with the Beljoxes Eye in Season 7 in which they find out The First could return because of the Ressurrection spell they performed on Buffy in the Season 6 Premiere. I always thought they meant it could return from Amends, but maybe this is what they meant. But then why wouldn't they use Hope's Dagger in Season 7?)

I think it's just a plothole in the game as it was made after Season 7, I think.

I Thought Only Kryptonite Could Hurt Superman. Not A Broken Heart.

From Beneath You, It Devours

The game came out before season 7.

The game is not part of official cannon obviously.

All Hail Joss Whedon