Hi Jen,

That £112 Travel Tax will just be part of a breakdown the cost, there should be other fees there as well which if you add them up will come to the cost of the ticket
Your e-ticket is your ticket so you should't have any extra fees to the airline.

Airlines 'codeshare' with other airlines, in the case of Etihad they codeshare with these airlines http://www.etihad.com/en-us/about-us/our-partners/
I'm fairly sure none of them operate out of Manila, but the easiest way to check is by looking at your flight number, if it's an Etihad flight operated by Etihad your flight number will be EY**** either 3 or 4 numbers after the EY