As I understand the situation, the Clans had very little info on the Inner Sphere pre-Dragoons. They had a pretty good guess as far as what was happening, but no firm evidence. When the Dragoons went in, they confirmed everything the Clans already thought. The Succession Wars had been absolutely brutal, and the Inner Sphere was close to falling apart.
What the Clans didn't know is that the 3rd SW acted as one long break for the IS powers to catch their breath. Very slowly, factories were starting to come back online. Jumpship numbers were starting to crawl back up. Trade routes were being re-established. The recovery was moving so slowly that you almost wouldn't notice it, even if you were paying close attention. But gradually the Inner Sphere made it back to the level where they could actually benefit from finding the Helm Memory Core.
The Dragoons stop communicating (can't count on those lazy Freebirths to do anything right), but the last time they checked in, the Inner Sphere appeared to be in an unchanging, idle state. So the Clans aren't particularly concerned about any of it. Their previous predictions regarding the state of the Inner Sphere had proven correct, and there's nothing to indicate that this is going to change over the course of a mere 25 years. Fast forward 25 years from the end of the Mad Max movies, where do you think those people will be? Then you get the Outbound Light, and they just show up above the Smoke Jaguar homeworld. Bad timing there.
The Jaguars (who always wanted to invade) reveal a bunch of partial truths. In reality, the Outbound Light was way off course. But the Jags insinuate that the Inner Sphere knows the route they took, and so it's only a simple process to figure out where they disappeared. They also have news that the #1 and #3 most powerful states have joined together, and nearly absorbed the #5 state. The botched War of 3039 is downplayed (it doesn't benefit your argument to show that they aren't moments away from re-establishing the Star League). But the Helm Memory Core would be a prime piece of information to release. They're rediscovering Star League tech!
The Invasion basically starts right then. Now the Clans know that the IS still hasn't fully upgraded from 3025 tech. How much they've upgraded depends on which version of the canon you want to pay attention to, but either way the Clans basically know that most IS forces are using primitive old tech. And while the leadership knows that the IS doesn't follow Clan honor, they don't really bother to tell their soldiers that. It's more like "pack your stuff, we're invading the Inner Sphere tomorrow". The average Clan warrior probably has the (non-combat) education of a 7th grader. They don't know what to expect from the Inner Sphere at all, because they've never been taught anything like sociology or the history of other cultures.
The Clans have the info in the Outbound Light's computers, so they know that the FRR exists, and they have a general idea of which planets in the invasion corridor are still inhabited. But they basically just show up and start shooting, and then ask their newly conquered subjects about what is going on in the area. They were pretty information-blind, but then so were the Inner Sphere powers. The Clans marauded through early opponents, and then mauled a ton of really elite regiments that were sent to try and stop them. Until Jaime Wolf's big reveal, the IS had no idea what they were facing. In game terms, their players didn't get to look at the Clan players' record sheets, or get to read the Tech Readout info on what their weapons could do. Imagine fighting a Black Hawk, and thinking "I'm at 10 hexes, I'm out of range of all those medium lasers he's carrying. I'm going to remain stationary and fire my PPC." And as IS units got wiped out, they didn't get to go back to their comrades and tell them what stupid mistakes to not make.
The Clans had what they considered to be "enough" information to attack. Their normal method of intelligence gathering is to just show up and ask what forces you have, so this was totally in keeping with how they had always behaved. It worked well enough because they so overmatched the people they initially fought. By the time the Inner Sphere and the Clans had figured out that the other side worked differently, the Clans were getting info from ComStar (and the IS from the Dragoons) and so they didn't have to worry about it.