I've read that for the 2013 Regal push-to-start, the t-harness is needed. Is this true? I have a manual transmission.
There is no T-harness for the 2013 buick regal Push-to-Start.
If the t-harness is not used, which wires actually need to be hooked up? Only the solid black lines on the diagram? I would of course be tapping into the factory starter (since it's push to start anyways). It seems as though some of the solid black wires on the diagram are supposed to be run to an aftermarket starter system, and not interface at all with the EvoAll (such as the +12V coming from the vehicle's pin 1 in the blue connector), which makes it seem as though I won't even be using all the solid black wires on the diagram either.
The EVO-ALL in the installation guide connects with an aftermarket remote starter, not the facory remote starter (which im surprised you would have if your vehicle is manual transmission). You need to connect ALL the solid black lines.
I've also read that the diode indicated in the instructions is not neccessary. Is that accurate?
If a diode is in the plan, you need to use it.
Finally, the instructions indicate that I need to cut a wire in the factory harness. Is this accurate? If so, is there any other solution that prevents the need to cut a wire? Warranty issues would prevent me from straight up cutting a wire in half.
Data needs to be cut. This is how it's done. If a T-harness was available for this vehicle, it would prevent having to cut a wire.
I recommend visiting a professional to get a system installed if you are unsure of what is happening in the installation guide.