Looks like you got it figured out. The diagram you shared is basically my new set-up, just without the 3rd set of elements. Here's a summary of my process of anyone else find themselves in a similar situation.
1. Bought a universal infinite switch rated for 240V from Amazon. They're less than $20.
2. Removed the timers and relay.
3. Mounted the new infinite switch onto the cover panel making use of the space vacated by the timers/relay. Just drilled a hole in the cover and popped it in.
4. Wired the switches directly to the kiln sitter and the elements. Just like in the diagram above: black terminal to L1 and white terminal to L2. H1 to one end of the elements and H2 to the other. The P connection on the infinite switch was left empty. The original switch stayed wired to the top elements. The new switch was wired to the bottom elements.
It's that simple. I was able to take the insulated wires that I removed from the timers/relay and use them to hook up the new switch, so that saved me from having to buy new ones. The new switch didn't come with a knob, but I picked up an old stove knob from my local Habitat for Humanity Restore for a quarter.
The trickiest part was learning how to balance the temperature from the top and bottom elements since the two switches are not identical and their range is slightly off.