1) Every different flight you take (i.e. each time you use a ticket coupon) you have used one of your segments. You are allowed four everywhere except NA where you can have six. You can buy two extra segments everywhere except your continent of origin. Stopovers are stops > 24 hours in a location. You can have as many as you want except in your continent of origin (Europe) where you are limited to two. Your proposed itinerary has four segments in Europe but you don't get back to CAI which you must do (see 3).

2) They don't.

3) Yes.

4) You can compute it from the data at http://www.consolidatedtravel.com.au/qf/taxes.asp . Figure on it adding 5-15% to the cost of your ticket.