Rather than trying to solder the rails together. Solder some jumper wires . 18 gauge should be sufficient.
I know DCS can be finicky at times. I’d still try to resolve the issues and only jumper the rails as a last resort. By over wiring the loop you may create more problems. You can check track voltage using DCS. I would think if the connections were that suspect you would see a voltage drop. It seems most of your issues are on the loop with a lot of turnouts. Are you powering them from a separate supply or track voltage ?
You still have 1 channel left in the TIU. Maybe you could divide the lower loop in half and see if that helps.
