You have the right idea, you can connect to J15 on the 410c, or you can connect to the test points (TP8-16) that you can see on the Schematic (page 30 of 33). Connecting to the test points is probably easier, they look like this:
The control hardware is also easy, you can use a FlySwatter2 or a similar device.
The last part of the puzzle is the software. OpenOCD is a good tool and connects the FlySwatter to gdb. I had a team create a project to upgrade OpenOCD to handle 64-bit processors, and the APQ chips in particular, but they are no longer with the company and not supporting this. However there is a speck of light at the end of the tunnel, all of the work they had done is on GitHub here: OpenOCD64. Unfortunately the changes are against an older version of OpenOCD (2015), and I haven’t tested any of this. Think of this as a bag of parts with ‘some assembly required’.
If you decide to take on this challenge, please keep @danielt and I informed as to your progress.
Full Disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other post may not reflect the opinions of my employer.