When you strip and sand down wood, the wood is still somewhat "sealed" from the previuos finish and will take stain differently than a new piece of wood will. It will look lighter because the wood is now more glass like.
If the color you was trying to get was not dark if enough even after several coats, then I would have just moved onto a different darker color. An espresso color was probably more in the ball park of what you needed to get the dark color.
5-6 coats of poly is way too much for the final look you was trying to achieve, even thinned 1:1. You should have probably brushed on a light coat of polyurethane varnish. Use long, even strokes, working in the direction of the grain. Once its dries, sand lightly with 320-grit sandpaper, then repeat and your done.
I feel even now if you sand down through the 5-6 coats as far as you can without luckily not sanding through, you will still end up looking "plastiky" because your table will look smooth as glass after sanding down and then having to apply another coat off your satin poly.
I also think your best option is to take the finish back off and start over. Remove all the finish, sand the top down with 150 in the direction of the grain, then I would use a wire brush (if your looking for that "grainy" look) and completely brush the entire top in the direction of the grain to try and open up the grain some, then come back with 240 and lightly sand the top in the direction of the grain. Try an espresso color on the underside of the table as Steve suggested and wipe some poly over it after the stain dries just to see if its close to a dark color that you are looking for. After you get the color right, apply it to the table top, then 2 coats of polyurethane varnish then see what you have. I think that will make the difference for you.