After I read your posts, there are missing points.

1. The nonce2 is only used to create next new local block header data. When a new block header come in, nonce2 is initialzed to 0 in cgminer, e.g. parse_notify() or gbt_decoode(). The nonce2 is 64 bits integer (very large integer).  The nonce2 only affects merkleroot value in local block header data. Huge numbers of local block header data are created and are given to ASIC chips for chips to find out some nonces which produce a hash value with last 8 digits 0.   
hash value: xxxx....x????00000000.

2. cgminer does not cycle nonce from 0 to 4.29G.  It just sends a lot of block header data to ASIC chips. ASIC chips or hardware do cycle nonces by digital chips and frequency pulse to check and find qualified nonces. ASIC hardware do filter of qualified nonces. So, ASIC hash header data super fast.

3. Qualified or found nonces, then, are read by cgminer. Then, cgminer verify those nonces with the pool's difficulty. Then, good nonces are sent to the pool for further verification by pool with network's difficulty.