I've been on the receiving end of a bunch of bad negotiations in RPGs. Real life negotiation training helps, but there's also some RPG specific aspects to keep in mind.
Often, the problem is that there's some adventure hook that requires the PCs to do something that's totally stupid. "Hi, you're level 10, would you like to go on a fetch quest for 100 gp?" Or the classic module Against the Giants, where the locals decide your high level party is guilty of unspecified crimes (vagrancy?) and must go kill 100 giants to atone. Even LG parties respond to that with "You're going to look mighty funny with that longsword sticking out of your ass." Your non-derogatory use of the term "railroad" (generally considered bad) leads me to believe this may be the case - if you expect the PCs to "take the adventure hook" you present them regardless of its desirability and they are trying to live in character and not go for some awful deal their characters at their current wealth and power level would never contemplate, you'll be disappointed.
Now assuming there's at least the makings of a deal, meaning each side has something the other person reasonably wants and might indeed exchange (goods, services, money, etc.) you need to remember that people want to make deals and that they can be win-win.
Allow me to use the framework presented in Getting to Yes, a book on negotiation everyone should read.
Separate the people from the problem. Put yourself in the PCs' shoes. Listen more than you talk. Get them involved. Help them save face at giving in on things. They should ideally be doing the same to the NPC.
Focus on interests behind positions. Ask "Why" and "Why not?" Be hard on the problem, soft on the people. How can everyone's goals be achieved? Don't attack the other person's position, look behind it. When they attack your ideas, ask for advice. Ask questions and pause. "How can we make this deal happen?"
Invent options for mutual gain. Generate a range of options, Use imaginative procedures. What other things does someone have that they'd be willing to do or give away that the other person might value more? It doesn't have to be a flea market "Five dollars! One! Four! Two! Three! Deal!". That's a one-dimensional negotiation. Timing, loot, favors, future goods are all possible. "What else can I throw in?"
Use independent standards. What is this really worth? If someone's being unreasonable, you can show what the item or job is "worth" to others. "These guys will do it for half the rate, but I want you to have the first crack at the loot..."
Develop a best alternative to a negotiated agreement. If one side has to have the deal or they're completely screwed, they are in a bad position. As a result most people have a plan B. As a DM, you need one too, besides "tell them out of character to take the bait." Maybe it's some kind of setup or extortion (e.g. the PCs' rooms get ransacked and a convenient clue is left behind pointing at the people the adventure hook is trying to sic them on. Of course the other negotiating party did it, but PCs are usually dumb and easily pointed in a direction.)
Of course, you can just let events transpire. In your example, the PCs got suspicious of the sage. So? Can they not get the information any other way? Are there no other people they can go to? Do they not plan to just stage a home invasion and interrogate him (normal PC response to something like this)? Your problem is less about negotiation and more about poor adventure design and railroading. Should there really only be one way to proceed in the plot? It's best if not; but if it is then the PCs will eventually have to give in or beat it out of him... I find you always get the best results from handling things in game, not with metagaming and railroading. It may seem expedient now but it's a long term poison for your game.
While it is true that 4e does center around combat, not all conflicts are about combat.
If you find some good resources on RPG plots, like S. John Ross' List of RPG plots, it can give you some ideas about running exploration adventures. Especially what he calls the Safari, Any Old Port in a Storm, and Uncharted Waters plots are good starting points to work from, as you want a focus on exploration. As for the implementation of this plots, remember your skill and social encounters!
The social encounter and the skill encounter will be your best friends. Social encounters are good for talking to "natives," maybe even talking to other random explorers. Skill encounters would be good for general scouting, climbing a mountain, opening a lost tomb, tracking a gryphon to its nest, and so forth.
Of course, you can always find a fight in the wilderness. Maybe the adventurers ignored the obvious signs of an owlbear's territory? Perhaps a random band of orcs or gnolls are inhabiting that ruin they discovered a few days ago?
As for generating the wilderness, I'm afraid using LOTR as a spiritual guide may be best. It also depends on what your setting is. If the adventurers are in a well-documented world, like those WOTC produces, then you could use those as a guide. If it's a home-made setting, then it's up to you how to populate that world. (Again, looking at a the WOTC campaign settings can be helpful in making your own.)
NOTICE: Subjective Input follows! I would recommend rough maps of places for adventurers to go to. This will give you an idea of what can happen in the coming session, and will likely produce a more cohesive wilderness rather than a nonsensical generated one. With such a planned wilderness, you can even have past exploration sites affect others. This means the ruins over there give a clue about the door to the ruins over here, or something along those lines.
Best Answer
If the combat is played for story reasons only and the character aren't actually willing to see which build wins over which, you could treat the face-off as a skill encounter.
This way, most characters would be able to participate without the need to find a seemingly impossible balance between roles and ranges.