Don't go spending any more money just yet. I've been down this road. Exactly the same happened to me.
Even though the battery-well is bone dry, you'll find residual water is trapped between the sheet metal and frame rails. (I created grid marks with some blue chalk and narrowed it down this way). Place some tissue down in both corners of the boot (including under the rearward edge of the foam cradle - there likely will be water under it). Go for a drive and make some fast hard turns. Take it up steep hills in both directions. Do this over the course of a couple of days, replacing the damp tissue each time and you'll find you have no more water down there - providing your seals are ok.
Try it out and let me know the outcome. I was in a similar situation, really frustrated, only to find it was the old water trapped between the sheet metal.
I left the interior boot trim off for a couple of weeks after replacing the seals and vents. I used a bucket, watering can and jet wash almost daily to ensure there were no leaks. It's absolutely bone dry and as good as new now. No condensation, no damp, no water. Better than when I purchased the car.
Place the tissue in this corner (the rearward end of the frame rail - you'll see there's a slight gap in that area, in the very corner, not visible in the pic) as this is where the water will leak from. Same on the other side, just lift the foam box up a little and slide the tissue in:
