It can be very difficult in a pen & paper roleplaying game to realize where the gap is between what you're imagining and what the player are imagining, especially since both mental images are coming from the same words. For example, one of the most frustrating roleplaying sessions I've ever played in (as a player) involved our party facing off against a major villain, who was busy making some sort of evil concoction in a foul-smelling bubbling cauldron. The battle raged on, the fighter fell victim to a hold person spell, the ranger was knocked out, and my wizard had run out of useful spells - so, in desperation, my character ran up and tried to push over the cauldron, spilling out the evil liquid and at least disrupting whatever ritual the villain was in the middle of. Or at least he tried to, only to be told, "You can't budge it. It weighs hundreds of pounds. [The bad guy] casts Inflict moderate wounds on you."

Remember, the players expect to be challenged, but they expect you to give them beatable challenges, and they will generally try their best to beat them in ways that make sense to them. I wouldn't have wasted a turn trying to push over a cauldron if I realized it was actually a solid stone unholy water font - which was what my DM had in mind - and your players wouldn't have wasted their first day on a Seven Samurai scenario if they hadn't thought that might have been a reasonable response to your scenario. That means your players probably underestimated what exactly a "massive horde" meant - did you give them numbers? Do they know how many people are in their town? 5000 orcs may seem beatable to a player that thinks that a "small farming town" has 2500 adults in it (as it might, in modern Indiana) and doesn't realize that in the middle ages that town has about 50 people able to lift a sword. If you realized this was happening, you could have just pointed it out directly ("hold on, guys, I don't think I explained this very well") or you could put it in to the mouths of the villagers ("There's no way we'll beat those orcs - we're outnumbered 100 to 1"). After all, their local lord ordered them to flee - so essentially every villager the PCs persuaded to stay behind was reinforcing their belief that "staying and fighting" was a realistic bet to make.

Second, as other posters have said, the sabotage attacks on the raider camp, and the moral arguments with their leader, were both good plans and the players had no way of knowing otherwise. Sure, you knew that the orc chieftain had received word straight from his god, but I doubt you mentioned that to the players - so their second night's work should have been rewarded somehow. I think this is where the players got frustrated - not so much that they guessed wrong, but that guessing wrong didn't get them any closer to an answer. I can see two possible ways to reward the players here, depending on how much you wanted to wing this adventure. If you decide in the moment to go the Seven Samurai route, then let the chieftain give his speech about his god, but let the players know that there seems to be some dissension in the ranks. Although the chieftain is firm in his belief, they notice small bands breaking off and riding home every so often over the course of the next two days - it seems their preaching did have some effect after all. Now they have to see if it was enough... On the other hand, if you want to stick with your original plan, have the orc chieftain let slip that no living man could dissuade him from his quest - he fears nothing that breathes - he knows he'll win this fight because the town has no necromancer - etc. etc. You may have to ladle it on pretty thick here. Remember that PCs will tend to think there's no connection between adventures unless you make it explicitly clear that there is one - your party probably has closed their mental file on that whole lich incident and is prepared not to think of it again unless you hit them over the head with it. In this case, though, you're trying to let the characters make that last connection - the chieftain is scared of undead - the lich! It's always easier in hindsight, of course, but if you can think of a few ways to have NPCs make the suggestion instead of just hinting at them out of character, it'll help preserve their agency.

Finally, just remember that the PCs know almost nothing of your plans, which means they can be changed right up until the moment they're revealed. And you can always stash them away for later. If it becomes apparent that the PCs think they've got a winnable fight up ahead against 200 bandits, but you actually had 1000 trolls led by a demon in mind, don't be afraid to just stash those trolls away for later and play the bandit fight the players are preparing for. You can always lead them back to the lich tower a couple adventures later.