MASSIVE Walnut Panel Glue Up - Did I Do it Right?! 

Great job

You can salvage Attempt #1 by cutting it apart on the seams. You will only lose a little bit and you can reuse the wood. More clamps, alternating top and bottom will help eliminate panel bowing. Also, you should not apply too much clamping pressure. That is the #1 cause of panel bowing; maybe # 2 after not having 90 degree edges. Echoing the other comment, clamping cauls will also prevent panel bowing. If you always build over sized, you could have eliminated that one pesky seam on Attempt #2. When you dry fit it and it doesn't close up, if you have extra width, you could have edge jointed that one board again. If you got it to close up in a dry fit, then it would not take so much clamping pressure to get the seam to close up. In that instance, if only you had allowed an extra 1/2 " or 1", you could have cut that board again and gotten rid of the rest of the bow, resulting in a good, tight seam. I'm not a fan of biscuits. I prefer dowels. They are stronger and give you good alignment. You might consider that. A doweling jig is less expensive than a biscuit joiner and I'm betting you already have a drill. There are numerous videos on this platform dealing with jointing lumber without a jointer. I recommend those to you. If you followed some of those recommendations, you might not have had that gap in the seam when you dry fit #2.

Try some cauls and more clamps next time.