iOS9 started defaulting to asking for a 6-digit pin now (though you can still do 4-digit). So it may depend on when the phone was setup.
Only for Touch ID enabled devices
No, it uses an exponential back off.
Even after the 'delete everything' ten attempts? In any case, you can just reboot the phone, and if it does write the number of missed attempts to NAND, you were going to revert that anyway.
Yeah I have no doubts there are ways around it - but they would definitely all contribute to making the brute forcing process A LOT slower.
Hasn't this phone been in FBI custody for months now?