I don't have any comments about how things will work in 2.0, only that I trust Francesco knows what mechanics to tweak to facilitate smoother gameplay. However, some general Encounter advice:
So is a success accomplished by some good roleplaying or by a good roll of the dice?
Both hopefully, so we tried rolling first and then roleplaying the result. That works well during travel & combat. But we still struggle during encounters and I still have to remind my players not to forget the rolling. Hmmm.
I like to let my players roleplay, and I pay careful attention for words and actions that challenge the personal interests of the NPC. This results in more Tests than Tasks generally, as my players also tend to simply start talking with the NPC and each other without proposing Tasks. After I throw some Tests at them, often challenging them in skills they are poorest at, they generally will shift their focus, get their head in the game, and realize that the NPC isn't simply sitting there passively, waiting for the right number of successful roles before giving the players what they want. My NPC is actively trying to assert their own interests, or reinforce their own perceptions of the players that they are dealing with.
Furthermore encounters are - 99% of all times - very easy to succeed. I can’t remember one encounter in our whole campaign, that went bad, so they didn’t accomplish their preset goal. There was no uncertainty like - will we manage to persuade Thranduil to help us?
No tension. Interest on the players side? Yes. Exchange of information? Sure! Hints to understand political aspects or contexts of the story. Absolutely. But no tension whether to reach the goal or not.
Two suggestions: use the optional rules for tracking the number of successes and create "tiers of success". Starting with something like "0-1 successes: players are chased off or forced to leave because the NPC doesn't trust the strangers trying to solicit something from them", and going up incrementally from there. Remember, in the twilight of the Third Age, people are isolated, ill-informed of outsiders, and therefore very suspicious. I often try to establish a sense of danger from simply talking to the NPC. Walking up to a travelling caravan? Expect the NPCs to put their hands on the hilts of their weapons and instinctively "circle the wagons" before even speaking to the players. Meeting with Thranduil in his hall? My players are certainly going to be disarmed, and Mirkwood elves will be in every corner of the room, watching the players' every move, gaze, and shifting of feet. This helps establish tension. I don't think you should have the threat of death in the forefront of your encounters all the time, but Thranduil is no fool, and he doesn't let outsiders into his presence often...
Overall, I think the key is to treat the encounter like a combat. NPCs aren't locked doors or hidden paths, simply waiting to be unlocked or discovered, they are actively pushing their own agendas all the time, same as PCs. Going back to your Thranduil example: how did they even get an audience with him in the first place? I would imagine there would have to be at least one encounter BEFORE the encounter with Thranduil to vet the players to see if they're even worthy of being granted an audience. If you do manage to talk to him, he's already self assured of his own abilities and wisdom, and of the capabilities of his people to handle any situation, so why should he even entertain your rag-tag group of outsiders? It would be in his best interest you say? How is it that you have some kind of inside information that he or his spies don't have, and what abilities do you possess that would cause him to trust you over, say, Legolas? He doesn't want your help, the same way you wouldn't be keen on listening to "outsiders" who are coming to you with information that you either haven't heard, or contradicts what people you trust are telling you.
If an NPC is already "on your side" or eager for assistance, it's probably not going to make for a good encounter, and is best left to either simple roleplaying, or maybe a simple pass/fail roll. The best encounters, like I said, are like combats: the NPC is trying to stop you from succeeding at your task, just like an enemy. They may not want you dead, but they certainly don't want their worldview challenged, or situations to change outside of their control, and they should be working to maintain the status quo that your players are trying to change.
"Of all the terrors that came ever into Beleriand ere Angband’s fall the madness of Carcharoth was the most dreadful; for the power of the Silmaril was hidden within him."
- The Silmarillion