1) Yes, you can definitely do this, you can create ads and just go into the ad details and change the ad author to the name of the other account you with the ad to be attached to.
2) This is not currently how things are set up to work as such, but there is a pause ad option in the customer dashboard. Essentially you could create the ad, and the customer has the option to pause the ad from their customer dashboard if they don't want it live, or the other way around, you put it in draft mode it, meaning the customer has to un-pause it to start the listing..
In terms of collecting all the information and posting the 100's of ads as you describe though, that in itself is going to be a big undertaking for you anyway.. good luck with that.