I know the answer and you are not going to like the solution...
I know EXACTLY what this is, I has this exact thing on our '11 3.0. You are right, it is the fuel pump. I had the SAME thing, fuel trim way off at idle and normal as speed up. Agreed, I had a hard time coming up a reason with why was RICH and not lean like most problems seen to be.... Here it is: The DIRECT INJECTION 2000 PSI HIGH PRESSURE fuel pump on these can develop leaks and leak raw gas right into the engine. Then the PCV system will draw out the fuel fumes from the warm engine. Problem is, this is extra fuel getting them burned so the O2 sensors see and fuel trims go way lean to compensate. But they go so lean it sets the codes you see. P0172 and P0175. At higher RPM, the leaked amount is small enough not to affect the overall fuel mixture even though still happening, but trims go back to normal.
The HIGH PRESSURE pump is driven off a cam shaft in the V. There was a service bullitten (#PI0145A) for the '10 3.0 to change the pump, but my '11 had this same problem so it wasn't completely fixed in the '11s.
Other things you may see: Open oil filler when engine off. Smells more like like raw gas than oil? Mine did.
Mine also DIESELED or RAN ON or what ever term you want to use where you shut off the engine and it keeps bucking and running for 2-3 seconds. NOT GOOD that so much extra fuel was in the PCV system the engine "ran" even once spark and fuel injectors were cut off. Didn't do this often, but it DID do it.
You are technically competent, as you own a Tech2, but here is the part you are not going to like: The pump isn't that much ($200ish from Rock Auto) BUT the service bulliten says to replace some high pressure output cross tubing. I couldn't find that part anywhere online. So, instead of leaking 2000 PSI fuel on a hot engine from a home repair, I had the dealer do it, as he found the pipe. Pipe was like $324!!!! Total cost :$716.
If don't do it, then I worried about fuel contaminated oil and the fact that high mileage 3.0/3.6 seem to get stretched timing chains, so I just paid the cash. FYI, had about 80,000 miles when started and finally replaced at 92,000.
Here you can see reference to the 2010 SRX, but doesn't say what to do
http://www.allworldauto.com/tsbs/NHTSA_2010_CADILLAC_SRX_technical_service_bulletin_339028.html
See what to do from : (Note, service bulliten says codes and running rough. Mine didn't run rough but for the RUN-ON and on long trips the LTFT would go back to "normal" so when come off interstate, the extra fuel wasn't "trimmed out" for idle soon enough and it would idle low and rough to the point had to use foot to keep running.)
http://www.terrainforum.net/31-engine-drivetrain/2128-just-noticed-fuel-oil-11.html
#PI0145A: Check Engine Light On, DTCs P0172 and P0175, Engine Runs Rough (Follow Diagnostics and Replace High Pressure Fuel Pump) - (Jun 16, 2010)
Subject: Check Engine Light On, DTCs P0172 and P0175, Engine Runs Rough (Follow Diagnosis and Replace High Pressure Fuel Pump)
Models: 2010 Buick LaCrosse
2010 Cadillac CTS, SRX
2010 Chevrolet Equinox
2010 GMC Terrain
Equipped with 3.0L Engine (RPO LF1)
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This PI is being revised to update the Warranty Information. Please discard PI0145.
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The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Condition/Concern
Some customers may comment that the check engine light is on, the engine is running rough and/or about an engine run-on for a few seconds. Upon further investigation, a technician may find DTCs P0172 and P0175 stored. In most cases, the crankcase may have fuel contamination.
Recommendation/Instructions
If this concern is encountered, perform SI diagnosis and repair as necessary. If SI diagnosis does not isolate the cause of this concern, then follow the steps below.
• Check the low side fuel pressure leak down with a mechanical gauge as outlined in Fuel System Diagnosis in SI. If fuel pressure leak down is present, with the J 37287 (Fuel Line Shut-Off Adapter) installed and closed, follow the Fuel Pump Replacement Procedure in SI to replace the high pressure fuel pump.
• Check the engine oil for fuel dilution contamination. Inspect the engine oil for a raw fuel smell. If present, remove the valve cover and inspect for fuel wash around the high pressure fuel injection pump plunger. Follow the Fuel Pump Replacement procedure in SI to replace the high pressure fuel pump. Also, change the engine oil and filter to complete the repairs.
Bob