I think you should be able to reach that back coolant temp sensor without taking the intake off, but you might need to remove the windshield cowling if you aren't taking off the intake. I have an SCV6 and did the whole job with the intake off, but I was thinking at the time that removing the cowling might give enough space to get back there if needed. Not sure though.
I think what ugmw177 was getting at in the above post was that, if you are going through the effort to change that temp sensor, might just as well change the crossover as well, i.e., take the intake off and change both the pipe and sensor more easily rather than struggle to replace just the sensor. Do you know if the rear crossover has been replaced in the past? If it hasn't been replaced, I tend to agree that it should be done and might as well do it now.
As for sensitivity to non-JLR sensors, my understanding is that the ECU compares the rear temp sensor (hot, out of the block) with the front temp sensor (cool, out of the radiator) as part of it's routine to make sure everything looks correct. So, I can imagine a cheap sensor in one spot could screw things up. Still, if you had the error before you changed the front sensor, the back sensor will likely need to be changed and I'd do that first - or maybe both simultaneously just to avoid having to bleed it twice.
Since this showed up right after a water pump replacement, are you sure the coolant system was fully bled out? Just thinking that an air pocket might prevent coolant from getting to the sensor. I'm not entirely sure that would cause this code, but seems possible.