Cisco has a nice IPSec Overhead calculator (CCO Login required, unfortunately).
From where we can draw, based on your IPSec settings and a few common optionals thrown in:
8 PPPoE (optional, but widespread)
20 outer IP header
8 NAT-T (optional, widespread, sometimes even default-on)
8 ESP Header (4 SPI ...
I found the answer further above in page 16 of RFC7296
There is no need to send a notification payload regarding a different IKE SA. The current IKE SA is already in the IKE header.
The first of these paragraphs in section 3.10 says "the SPI is included only with INVALID_SELECTORS, REKEY_SA, and CHILD_SA_NOT_FOUND".
Section 1.3.2 on page 16 makes ...