Luke knows more about this stuff, so he may have a different answer, but IIRC the new plane types may not show up if you still have a full complement of the old plane types left on that date - they may not start showing up until you have some losses. At least, I think I remember seeing posts to that effect some time ago. Maybe try damaging some of the N17s on the ground in a mission and see if some Hanriots show up as replacements?
That's correct - sometimes you won't get a new aircraft type on the scheduled date if you have a full complement of aircraft already.
Actually, you have to lose a pilot (with his plane) in order to eat into the supply. Crashed planes get repaired and stay in the squadron. It would be better to start a career AFTER the plane switch.