the ALM on the trimotor, keeps an eye on the trim during the flight, backing up what is displayed on the AIMS
There is no actual requirement for him/her to be on the flight deck at all during AAR, although they do have an excellent understandng of the what indications should be on the AAR panel during AAR operations and do provide valuable prompts at times
During ops they are also invaluable when it comes to digging out codes etc that might be required.
occasionaly if the AE is particularly busy or very lazy then the ALM helps out gathering weather if required
Regarding use of the stewards during local flights it has not unknown and is authorised for them to fly as the only non flight deck crew during MCT sorties