That was a lot of questions! I have bred neon tetras, which are similar in their habits to zebra danios, so I will try to help.
Whenever a female decides to drop her eggs, the males will want to be there. It is in their nature. So yes, if you have a male with her, he will fertilize her eggs.
You should probably keep the males and females separate during the conditioning stage. My tetras were constantly chasing each other around, driving me and them nuts. You do not want a stressed out female, so you should probably keep them separate. Give them plenty of live foods!
You can fairly easily tell when a female is ready. Don't look at her from the side-- they often look fat just from food that way-- look from above. If you can see a significant bulge on either side of her, she is probably heavy with eggs. Some females are fatter than others when they are read to drop their eggs. I had one tetra that was as skinny as a bone but actually dropped a lot of eggs. Just condition the females for about a month and then introduce the males.
You should probably introduce more males than females. This way you can be sure that one of the males isn't sterile and all your efforts are for nothing.
One last thing-- consider adding a spawning mop and putting down a layer of marbles on the bottom of the breeder. The danios will lay their eggs in the mop, and any of them that drop will go in between the marbles so the parents can't eat them.
I hope this helps!
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