First, don't cater to such a wide spread party size. Make a decision and base encounters on that decision. If this was just a single player game, basing encounters on one player character would be fine, but since they are making this game with multiplayer in mind, to make the game well balanced, encounters from the beginning should be based on five characters. The game should allow you to create four custom characters with default character designs so that if the player only wants to create one character, they can do so and just accept default custom characters. Then, before the first encounter, you add Lae'zel to the party for a total of 5. Us should be a severely nerfed intellect devourer so that it is more of a companion newborn and not carrying the party as a full blown intellect devourer.
With this in mind, here is how the combat encounters should be. Note, the intent here is to create appropriate encounters based on level and standard party size while providing a variety of monsters to add more flavor to the game. If you, the player, don't want to play the game based on a standard party size, that should still be an option, allowing the player to choose to not have any other characters in their party if they want. In other words, if they want a challenge and want to solo the game, they should have the option to not even have a party of 4 to begin with. But again, the encounters would be based on a party size 4+ from the beginning. Also, using standard 5e stats.
First encounter = 8 Manes, AC 9, HP 9, +2 to hit
Second encounter = 1 wounded imp with 4 HP and 2 Manes. This would be an optional fight on the top deck.
Third encounter = intellect devourer with 2 HP and 3 thralls with 3 HP and 10 AC and + 2 to hit. This would also be an optional fight on the top deck.
Fourth encounter = 3 thralls same stats. Optional fight only if you press the wrong button on the machine near Shadowheart.
Fifth Encounter = intellect devourer by itself with 4 HP. Optional fight if you ticked off the collective by fighting with the previous intellect devourer or you killed Us.
First helm encounter (based on Shadowheart now being in the party as well, since it is likely that many will not just leave her. If they do, one less character shouldn't affect the balance too drastically) = 1 wounded imp with 2 HP, 1 hellshog, and 4 Manes with 3-5 HP each.
Second helm encounter (this should be like a final boss fight and should be more challenging. It should be almost like a capture the flag fight instead of a last man standing fight. In other words, your objective is to just get someone to the helm. It is NOT to kill everything. As it already is in the game, Lae'zel and the mind flayer urge you to just focus on that objective so you know killing everything is obviously not your goal. If you do, kudos, but the encounter should be built with this objective in mind. Simple AI attacks whoever is closest to the helm) = 2 imps at full health, 1 dretch with AC 11 and 9 HP because wounded, 1 hellshog and 8 Manes with 4-9 HP.
Beach Nautiloid Fight = 1 wounded intellect devourer HP 5, 8 Neogi Hatchlings AC 11 and HP 1-7. Fight assumes 4 PCs + Shadowheart. Shadowheart tips you off to stay at a distance from the intellect devourer. Again, simple AI. Attack closest.
