All vendors have lighting planners, follow it.

"However at the first junction box it is split into 12/2 cable feeding some outlets and 14/2 cable feeding the lights. Is this a problem?" Yes, if you have correctly described the situation, it IS a problem. There are some very narrowly defined circumstances in which a short length of #14 wire might conceivably be used for as a fixture whip on a 20-amp circuit, but at first glance, your description does not appear to fit those circumstances. Another issue that could be in play has to do with whether the "outlets" fed by the 12/2 cables are small appliance receptacles (e.g., along the kitchen counter, a work island or dining room). If that's the case, regardless of wire size, receptacles and lighting cannot be on the same circuit.

You can't legally do electrical work in a multi-unit dwelling. So let your electrician handle the details.

I understand the you cannot use a bare copper wire to carry any loads so it CANNOT be a neutral but what if the existing wiring to a detached garage is 3 wire all in a heavy insualtion and the black, white and GREEN wires are all insulated as well and are all 6 guage? Can I convert that GREEN wire to the neutral since I won't be running a ground out the new subpanel anyway? I am not trying to be cheap about this at all, its just that the previous owner who ran the exisiting wire did so in PVC which is now buried under the driveway and there is no easy way to dig this up or run a new line. Since it is the same guage as the black and white wires cannot I use colored tape or something to mark the wires in the subpanel so no one in the future thinks its a ground wire? Thanks, Bob W.