2006 prius Red triangle, loss of power

Hello
Having some issues with my used prius hopefully someone can help.
Background:
Purchased a used 2006 prius with ~180K miles on it from south bay a few months ago. At the time of purchase the guy warned us that it 'ate' motor oil.
Symptoms and actions taken:
After 2 months of driving without issues (driven couple of hundred miles), on an uphill road the engine overheats and throws the red triangle, vsc and check engine lights on the dash, along with loss of power. After cool down I check oil level and sure enough it is low, since this is on top of a hill we essentially coast all the way back home without any real issues. Once home I top up the oil. I do a test drive around the block without issues (no loss of power) though the check engine/red triangle/vsc lights are still on. But after I take it for a longer drive the loss of power returns.
Since my bluetooth obd2 connector wasnt working I borrow one from my friend and try to get a read. Unfortunately the thing puts out no obd2 error codes. Living in the bay area I apparently cant just take it to an autozone, to get code readings. I take it to meineke auto repair, they too are unable to get an obd2 read, and want to charge me $100 for a full reading, which I decline.
I do a few diagnostics steps including 'resetting' the computer by leaving it off of the 12v battery for a few mins, this clears the red triangle et al on restart. I do a test drive, at first everything is working just fine, but after about 10 mins or so of driving the red triangle/check engine/vsc lights returns, along with loss of power. (under ~20 mph only, cant go uphill)
Take it to a mechanic and they come back with code: "APOA92. Trans Axel issue - MG1 performance is off. Measured torque is different than executed torque, enough to be a problem. 95% sure it is the transmission." (My partner got this over the phone from the mechanic, and emailed it to me). They are quoting around $5000 for a transmission installation parts & labor.
I stupidly still havent checked the 12v battery.
Questions:
Do you believe this is a tranny issue? Any point in me trying to double check the 12v battery? Should I try to get a second opinion from somewhere?
Any other questions or comments?
Thanks

 

bisco, do you know what the cost of something like that would be?

 

ooph reluctant to spend something like that on this used car, considering I dont know what other issues might prop up later on. I was hoping it would be something like the trans fluid pump, but if the consensus is that the whole trans needs replacement... I'm not sure that's worth it.
Thanks bisco.

 

I suppose I'm trying to get more of an assessment on whether this transmission replacement is needed. Does that code show the need for a transmission replacement? What other possibilities are there? What about he transmission fluid pump?