You're getting plenty of guitar suggestions already; I'll only add that my choice in this area has been the GS Mini; about the only place it fails your checklist of requirements is it's a short scale with a more narrow nut.
I'll also mention having seen a travel guitar somewhere that could be broken down (i.e., the neck came off of the body) into a more compact size, packed away inside it's case, then easily re-assembled for use. I don't remember the brand, though; I don't how well it actually works in practice; and I never had a chance to play it to see how it sounded.
Switching to travel-related issues: I know that the rules domestically say that the airline must accommodate musical instruments as carry-on luggage IF there is space available. So, being cynical, if they claim it's a full flight and there's no space available, that's their way to dodge that requirement. That being said, the one time I've traveled with a guitar (domestic US carrier, probably about 7-10 years ago now -- and before, as "Guitar Cowboy" points out, the airlines started nickel-and-diming folks for every little perk like pre-boarding...), the gate agent and crew were actually quite helpful: the gate agent saw me standing there with a guitar case as I waited for general boarding and offered me pre-boarding; and the crew let me stow it in their closet up at the main cabin door. Keep in mind that the rules for international travel (or domestic carriers abroad) may be different; you'd need to inquire.
I'm also not sure what the rules for carrying Humidipaks inside the guitar case would be if it was brought on as carry-on luggage. TSA is fussy -- and also quite inconsistent from airport to airport -- about what they'll allow past the security checkpoint; and oddball things like Humidipaks tend to send them into "I don't know what it is, so it shall not pass..." mode. Plus it's never been entirely clear to me what category Humidipaks would fall under on the TSA's "YES-NO" list. But given they won't let passengers bring a bottle of water past the security checkpoint, I'm thinking whether it's considered a packet of water or a "gel substance," its chances of getting past security are low unless it falls below the minimum requirement for carried-on liquids & gels... I personally wouldn't even bother with Humidipaks; or if I was going to be away for an extended period to a particularly arid part of the world, pick up one of those humidifiers that just has a sponge in it, and that can be wetted with tap water as necessary.
Good luck and have a safe trip!
