Ref APU Fuel usage, it depends on the APU.
THe old Garret GTCP85 which was on most old jets was most economical with the bleed off.
Most modern APUs have a separate bleed compressor which runs all the time. With the bleed sw off, the surge valve will open and yes the fuel consumption will get higher.
On my old Tristar, the most economical was APU in min mode, bleed valve open and one pack sw on. This definitely used less fuel than switching off the bleed valve.