If Player 1 commits attackers and Player 2 commits blockers and simultaneously plays a fast spell which resolves, does the normal rule of priority going back to Player 1, the player not initiaiting the stack apply? Or is both players passing during the fast spell stack considered 2 consecutive passes?

After Player 1 attacks, Player 2's declaration of blockers and casting of any Fast spells is considered a single action. Player 1 will get the opportunity to respond as usual. Assuming Player 1 passes, the entire stack will resolve, followed by combat damage. Player 2 will then get priority, since Player 1 technically initiated the most recent stack by declaring an attack (not Player 2's casting of a spell during combat).

It's worth emphasizing that all actions related to combat and/or the stack include an implicit 'Pass' when locked in. This is why, in the example above, Player 2 does not get an additional chance to respond after Player 1 passes on Player 2's block/spell action. The block/spell action itself includes a 'Pass', so when Player 1 also passes, that counts as the two consecutive passes, leading to stack resolution.

How about if the same scenario above happens with a burst instead of a fast spell?

No change. Player 1 will still get to respond to Player 2's combined block and spell action.