[Groth]

Woo hoo, my car stops now. Slide hammer - fun toy.

Do you know of any good schematic capture shareware? I don't have any good way of creating schematics at home.

I've been using LTSpice. Linear puts it out free to encourage the use of their switcher components. It isn't the greatest Spice implementation, nor super for schematics, but the price was right and the learning curve agreeable. I've also been trying to forge my way through the free version Vutrax, with little result.
At home, eh? What packages have you at work? Perhaps they have a cut down share-/free-ware version.

I hope you don't mind some suggestions...

Bring it on...I've more enthusiasm than knowledge and experience.

I'd go for minimal TCR. Actual resistance can be measured and compensated for in the following gain stage. Drift due to variations in ambient and airflow can be a real pain. I don't know what value of shunt resistor you were going for, but keep in mind, the larger the shunt resistor, the more of your Vcore adjustment range you are giving up. On the other hand larger shunt resistors will give better signal to noise ratio.

I lack the equipment to accurately measure resistances below a tenth ohm (&$^%# DMM), making 3% tolerances mighty ugly. Cheap upgrades? I could have fairly wide uncompensated temperature swings with the series 10 before error reached 3%.
On the K6 board, I discovered that the regulator was not compensating for the shunt - the nominal 2.4 V was appearing on the inductor side, not the CPU side. I'd like to think that KX7 has better feedback circuits, but, to be safe, I'll use a shunt in the 0.5 to 1.0 mOhm region. This will also decrease the self-heating and associated drift.

I'd suggest adding a film 0.1 uF cap in parallel with the shunt resistors. This will bypass a lot of the high frequency transients that come through the capacitance of the switching inductor. These transients might screw up the amplifier output somewhat, unless the amplifier is fast enough to track them.

I used a RC low pass filter on each side heading into the difference amp. 1 Mhz corner frequency versus 3 Mhz unity gain bandwidth. I'll add a shunt bypass cap, and see if it anything changes.

I'd suggest using OPA227 Op-Amps. (OPA2227 or OPA4227 for duals or quads) With your present setup (20 mOhm shunt, 9A max current) the input offset voltage of the LF412 can cause a +/-1.7% error in the first difference amp alone. The main advantage the LF412 has is low bias current, and because of the extremely low output impedance of the shunt, bias current is a nonissue. The OPA227 has substantially better: input offset voltage, input voltage noise, common mode rejection ratio, and unity gain bandwidth.

A case of using what I had in stock. As for the input offset voltage, a bit of hand trimming while using known inputs killed it. Power supply noise/regualtion was problem; cured with 10V zeners and filter caps.
I'm more than willing to pop some OPA227s in to compare performance. If nothing else, I'd be a fool to turn down parts at bulk prices. Surface mount is wonderful, saves tedious drilling.