I've done all ways.
On our first visit home back to Oz with the twins I took cloth with us, but did dispo's on the flight.
Second visit home I did disposables
Third visit home i did pull ups for the flight and at night
Fourth visit with Lola I did disposables all the way!
Maybe that progress tells you something but in all honesty a 18 hour flight IMO is not feasible for cloth. theres a few reasons. 1. storage that means another big bag that needs to go on board, and where there isn't much room as it is, why make like more difficult. 2. you HAVE to plan for delays etc, so would need to take at least 12 hours more nappies. Thats a lot of stuff to take on board with you and cart around.
As for the security guards if they want to inspect your poopy nappies let them! thats their business.
But honestly i love using cloth, i wouldn't change it for anything but there are times when its a case of why make like harder than it is. Now using cloth isn't hard, but travelling with a baby on a 18hr flight isn't easy.
As for the time away, if you can keep up with the washing, and whoever your staying with won't mind, then go for it. But it will add up to a fair chuck of luggage.
But I would take a 3 week break and look forward to it when you come back. It can be done, I've done it with twins, but wouldn't do it again.