[5thgenmaxima]

Refrigerant is very cold to the touch..
1.) If your evap core or a hose blew it probably wouldn't leave that much fluid on the ground. R134 has a boiling point somewhere far below freezing (or below the temp in your car) when not under high pressure.. By the time the line blew the refrigerant would have boiled (evaporated) away and you most likely wouldn't have been left with a puddle....maybe a florescent greenish color from the A/C dye..But not enough to cause a huge puddle.
2.) Your A/C having an issue would NOT cause your car to overheat. It would also not cause your radiator to be empty..You said after you topped off your radiator you started getting more fluid on the floor...That pretty much proves that it's a heater core.

If you have to get home or to a shop you can take the two hoses going to your heater core from under the hood and run them into each other like stated above..

I don't want to rain on your parade, but you're probably better off taking this one to a shop.. Interior work is a major PITA. I fix cars for a living. Anytime a dash rattle, heater core, etc comes noone wants to do the job..
This kind of work sucks.. I don't know about nissans, but if the WHOLE dash has to come out in one peice if gets tricky. You have to have the dash ready to be removed. Then you get it unbolted and get a friend to help you pull it out of the car... But watch the door seal and all those small EXPENSIVE pieces of trim.. Make one wrong move while pulling the dash and you end up with scratched trim pieces, a ripped headliner, messed up shifter knobs, chaffed wires or just scratches on the dash...I don't know if nissans are as hard, but in some cars you literally have the heater core inside the back of the dash. You have to take the dash out of the car and then remove the heater core from the back of the dash....It's a huge PITA... If the mechanic scratches the trim or dash then the shop is liable to replace it. If you scratch it, then you have to pay for all that stuff yourself... If there is a dash rattle when you're done you have to tear it apart and find the rattle. If they create a rattle they have to fix it for you...Not to mention that if you get 1/2 way thru the job and decide you want a shop to fix it, a shop won't dare touch the car until you put it back together so they can remove the parts themselves..
I'm not trying to be a d1ck, I'm just saying if you're not knowledgeable enough to know if it's coolant or refrigerant I wouldn't attempt to tackle this job....If you do decide to tackle this job, good luck.