Since you're gestalting to compensate for small party size you should be gestalting for maximum diversity.
Remembering that Kingmaker is a hex crawl, Swash adds very little to Bard. You have enough performance duration to be an effective combatant. Swash lets you skimp on your physical stats for charisma with the BAB and HD advantage being worth +2 each to your attack stat and con and Swashbuckler Finesse letting you use dex in place of strength (at a damage disadvantage, but an acceptable one). You're not really gaining any new capabilities, though.
Just for starters you could do daring champion cavalier. You keep the panache, get tactician and challenge, and get better saves. But you could be picking up supplemental divine casting as an oracle or paladin or wider arcane selection as a summoner or eldritch scion magus as well.
Brawler/investigator is a good plan. Investigator is really good with any full BAB class that isn't charisma based.
Druid/rogue or druid/slayer can maybe be made to work solo, but it'd be a fog based build and not very party friendly. You do have the option of making everyone take a druid archetype or oracle mystery that can see through magical fog and be absolutely terrifying, but that's probably more coordination than you'll actually manage to pull off. You'll also suffer from a lack of condition removal unless the bard pairs with oracle. Druids do enough that oracle can handle the rest without overstressing their spells known, but you need someone with cleric list casting. Reincarnation is a crapshoot. I'd suggest straight cleric/slayer or cleric/ranger or even cleric/rogue. A battle cleric build isn't bad and adding sneak attack isn't going to actually make anything worse. If the bard is pairing with oracle and you're not doing the see through fog party slayer is better than rogue but you'll miss out on the combat style stuff because the natural weapon style is terrible.