My current guesses in regards to the reels:
There are five reels.
The break between Reel 1 and Reel 2 is between Esteban and Joe meeting for the first time and the arrival of the cavalry into San Miguel.
The break between Reel 2 and Reel 3 is between Joe warning the Rojos about the supposedly escaped soldiers and the Baxters racing to the cemetery before the Rojos show up.
The break between Reel 3 and Reel 4 is between Joe shooting up the small house and some nearby Rojos reacting.
The break between Reel 4 and Reel 5 is between the initial explosion of the Baxter mansion and Ramon signaling for the wine barrels to be brought over to use as fuel for the fire.