Who's yelling? If I were to yell at you about something ... trust me ... you'd know it

I'm just saying that some people are comfortable in doing this kind of work and some are not. No harm in it either way, and for those that are not fully at ease with this stuff it is good advice when someone suggests to find an installer that is. For me? I know what I
can do ... and what I'm
willing to do. For example - I know how to hang and finish drywall. However, if we were to do a large remodel project that required pallets of dyrwall to be delivered and hung (like finishing our basement) - just pay the man 'cause it is NOT worth it to me for that much work
Moral of this story is even when I am comfortable doing the work, there are times that it still make sense for me to pay an installer.
Anyway ... back to your questions.
The "extension" harness that is included with the rear saddlebag lid speaker is the same harness that is included when adding rear speaker pods to a tour pak. I'm calling the "extension" harness the harness that runs from the fairing down the back-bone. Because it is the same harness, there are additional wires (4 per connector) that used for the controls that are present in the rear pods (headset vol, track up/dn, etc) They are not used in the rear sadllebag lid speaker install.
In the NUOH harness, you can cut, de-pin, re-terminate with additional connectors, etc the 4 wires you need for input and output of the 70.2 amp. Whatever method you like. Makes no difference if the connectors themselves are mated as the additional wires are used for rear speaker pod controls (see above). You could, of course, build an interface harness that would route the speaker signals through the 70.2 amp and would plug in to each side of the mating connectors you already have. The bottom line is you need to route the rear speaker signals
from the NUOH harness
to the input side of the 70.2, and the output signals [from[/i] the 70.2 amp
to the extension harness running down the back-bone (just as you have it drawn out). The minutia of how you accomplish it is up to you.
Not sure about your reference to "3 wires per speaker". I don't recall seeing that but if there is such a reference, it is a mistake.
Hope this helps ...