First let me say you have to get the boards flat or its going to be a fight all the way through the glue up and you will not end up with a good result. Start by using winding sticks to see what the twist situation is. This can be as simply as a couple levels, or 2' long straight boards or aluminum angle. You've already got an idea using the string.
There are 3 ways to do this, a power jointer, a router sled or by hand (which, of course requires hand planes). If you use a router sled you'll need a planer bit. They come in various widths. You can do it with your PC just make light passes and let the router cool off as needed. I wouldn't go over a 1 1/2" bit.
Once you get one face flat, plane the opposite face to get uniformly thick board. Then you are ready to proceed with jointing. BTW, your wood is well acclimated, but nevertheless do not EVER leave the board laying on the table, IOW always be sure you store on stickers.
On jointing, having a gap at the ends is not good. Having a gap in the middle is acceptable (its actually a spring joint). I think you're router technique for jointing might work, but I've never done it that way.
IMO whichever way you go, the only way to really dial in the joints is using a jointer plane