Doorbells are not required when sending NWRITE packets, they are simply a method to let the sender generate an interrupt on the receiving device. A common usecase is to notify that RX device that data has arrived (via other packet types).
Your other questions are answered in your other thread...
http://e2e.ti.com/support/dsp/c6000_multi-core_dsps/f/639/p/311852/1087992.aspx#1087992
Regards,
Travis