Interesting argument, Berf.
I freely admit I lack the experience with solitaire to really know how it feels in play, but the math suggests to me that 2-player should be tougher than single-player.
The number of player marines and actions increases by 33% going to 2-player (from solitaire), while the average number of genestealers per room goes up by only 32% (once taking into account the weighting of the spawn-types in the deck), so, yeah, in that regards it's a tiny bit easier on the players.
That said, a major spawn that goes unanswered has at best a 50% chance of killing a marine in solitaire, and a 66% chance in 2-player, so in theory at least the 2-player game should be less forgiving. To get accumulate a single super-swarm (one that kills automatically unless mitigated by a special power) all you need is to get major spawn on the same color two turns in a row, even if you kill one from the swarm in between. In solitaire, it would take 3 such turns in a row, which is both significantly less likely to occur, and also gives you one more turn to roll a skull or two and defuse it before it gets ugly.
Given those factors, it would seem to me that having one bad turn or making one play mistake should be far easier to recover from in solo play than in games with more players. And there's certainly more chances to make such mistakes in multi-player games.
The Instinct mechanic starts to make these bad choices more likely once you add more players into the mix. With one player, you've eliminated the dangers associated with incomplete information or selfishness.
You add a 1C room to the mission as well, adding 25% to the length of the mission (another 6 to 13 genestealers you have to kill before the game ends, and several extra event cards to draw), which should also increase the chances of losing the war of attrition.
Lastly, with an 8-marine line-up, I would expect it gets trickier trying to get good weapon coverage. A 2-range marine can theoretically cover 40% of the battlefield in a solo game, but can only cover at most 31% of the battlefield in multiplayer (at least until marines start dying). In some locations that's more important than others (with spawn points more densely arranged or more spread out), but it's yet another thing that incrementally complicates multi-player games. In a solo game, a marine with nothing better to do is closer to the Door or other Activate-able terrain, but with more players it's increasingly likely that the characters who could be spared for Move+Activate are at some far end of the line where they can't get to the relevant terrain cards.
I would expect all that would add up to slightly more difficulty at 2 players than at 1.