oooh i didn't see the reversed battery, sorry. anyway it just has the effect of cancelling one of the batteries going the other way. You can think that section as like -1.5V +1.5V +1.5V +1.5V and there's a net voltage supplied of 3V (equivalent to 2 batteries). That's probably a key step in solving the other problems.
This is an interesting electricity question relying on some nuances of Kirchoff's voltage laws that aren't really studied in year 12, so while it may be in your current course Adequace, it's certainly not something that other people should be worried about.
Long story short, one of the batteries being reversed just takes away from the total voltage that is being supplied and it's as if were were only 2 batteries there.