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If you don't need the extra 180mm, then any expense is wasted. You really won't know until you get there, that's the problem. To a degree, you can gain some insight thru the experience of those who have been there, but keep in mind shooting styles vary. A hardcore wildlife photog would likely say don't bother with anything under 600mm where a landscape guy would say anything over 20mm is to much. Depends a lot on what you want to do.
As said, forget the TC for your lens choices, even if it would physically fit the IQ would be horrid.
Don't worry about the overlap, it may save you from having to change lenses constantly. Another thing to consider is weight, do you want to travel light? If so, you might just bring the 18-200. For me, I like the option of going wide, if I were going to Alaska, I'd bring my 12-24 for sure.
I'd take the 18-200 and if you want more, I'd rent a 80-400 for the trip.