Resolving two 3-Way Switches for two separate lights on one Combination Switch
Check your box fill
Your plan to feed the recessed lighting from and mount its dimmer at the existing two-gang where power comes in is a good one, save for one issue: box fill.  You see, the NEC allocates a given amount of space for wires and devices in boxes to avoid overcrowding and its consequent issues with wire-bending and overheating.  In particular, you have 18 wires in that box, consisting of 12 current carrying wires, 2 wires' worth of grounds (1 for every 4 grounds), and 4 wires' worth of devices (2 wires per gang * 2 gangs).  For 14AWG wire, this yields 36in³, which is a bit more than your average two gang plastic box holds, as those cube out at 32in³.  So, you'll need to check the fill capacity printed on the inside of your existing two-gang box, and replace it with an extra-large box if it comes up short.