Re: 2 ?
Does the Guest card only give you the extra family member for one turn?
Yes, that's the way I read it.
With the Manure card, does a player get to take 2 from each field on a Harvest Phase, or does the one per field that is provided by this card only apply on non-Harvest rounds?
"Harvest [fields] at the end of each round... even if it does not end with a Harvest". The intention appears to be that you get one item off each sown field on non-harvest rounds. Harvest rounds would be handled normally - I don't think you get two items off per field! The specific wording "you get 1 grain/1 vegetables from each of your fields at the end of each round", also seems to exclude getting TWO items per field in harvest rounds. (Nice card to get, if you can manage it early enough!)