Remodel Leaking Drain Emergency

I am finishing a remodel and set up for tile. Had vinyl shower pans installed in both showers several months ago. They held water, passed inspection and so I cut out the drains to let the water out. So much has happened since with workers walking over everything that I decided that I wanted to retest. Used a drain plug in one drain without a problem but the other shower drain is old and the threads on the inside are rusted off so I cannot screw a nipple down into it. I tried using a drain plug but because of the uneveness of the surface water still finds it way down the drain. I am at a loss. I don't really want to crack up the slab, change out the drain and have to put in another shower pan, etc. The trap is full of water which implies to me that the drain is holding water. A guy I know came over and pumped the line with balloons and said that that the pan was OK but the drain seems to be seeping but if so the trap would be dry. What can I use to plug that drain and check the pan other than the conventional expandable drain plug. Are some drain plugs better than others? How do I install the drain plug, maybe I am doing it wrong. And lastly, is rust on the inside of the collar where the drain and sub drain connect with deep crevaces a problem. Redoing this drain and shower pan would be $$$$$. Help if you can. Thanks.
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