For I-765:
(c)(9)( ). This is the general category for applicants for Adjustment of Status under INA 245, doesn't matter if it's via marriage (I-130 or I-129F), other family (I-130), work (I-140), investors (I-526), special immigrants (I-360), S-visas (I-854), T-visas (I-914), U-visas (I-918), or Diversity Lottery. There are a few groups that adjust via different sections of law and use different codes (asylees, refugees, and pre-1972 registry applicants).
Do not use A6, A6 only exists due to a quirk in the law where technically K-1 visa holders are eligible to work incident to status, however that is only valid while the K-1 is (so 90 days), so that will in the absolute best case scenario only get them an EAD valid for a couple of days and more likely the K-1 period will expire by then and the I-765 will just be denied for mootness. Again, use (c)(9)( ).
I-765, #24: K-1 for the husband, K-2 for the stepdaughter.
I-765, #25: Same as above, if their admission periods expired "K-1 expired" for husband, "K-2 expired" for daughter. In the event you'd file I-765 after I-485 that's when you can alternatively put down "Pending I-485", or "Applicant for Adjustment of Status". Ultimately USCIS is not too picky about what you put in there as long as it makes sense.
I-765, #21.b. - Put down whichever passport he was admitted on, in this case it'll be the newer one. It's not an uncommon scenario for the visa to be physically in an old passport.
I-765, #21.c. - Leave blank, this field is for cases where you enter US on something that is not a passport (e.g. Any WHTI Document (Canadian Enhanced Driver's License, NEXUS, Sentri, etc), Canadian birth certificate (if under 16, or under 19 if traveling with a school/religious/youth group), Border Crossing Card, US green card, Refugee Travel Document (US or otherwise), Re-entry permit, UN laissez-passer, maybe 1954 convention stateless travel document (not sure, US never signed that one) and probably some others). In any case, leave that one blank, or if it being blank bothers you just put in "n/a".