New Home being built - questions for the pros:

1. if my builder runs the combustion air/exhaust vent up one of the flues in the chimney (the boiler apparently has a coaxial vent system which brings in air and exhausts the gasses through the same pipe), how does the combustion air not suck in the smoke coming from the chimney from the other flue for the fireplace?

2. we will have a a.c. condenser in the attic and the HVAC guy is reluctant to put in a humidifier because of freezing and leaks. Is this a reasonable concern and if so, how would you recommend that we humidify the air upstairs?

3. makeup air for the fireplace. i understand i can have a mechanical system to provide, have a vent inside the fireplace (i have heard from others that this does not supply enough air and that the cold air does not combust as readily) or just open a window. of the first two options, does anybody have a strong opinion? i have searched the forum but cannot seem to find conclusive opinions. just opening the window will not be something that will work for me.

Thanks for the input and my apologies if i have used terms incorrectly here.