The lift gate button has 4 push on clips. As soon as you got couple from top undone, the bottom two will take a second. You need to stick your hand inside to loose the switch. You can as well just unplug the plug, but found removing the whole rear gate switch makes it easier to work on the project.
No cable splicing. I have showed the two wires into the plug to make proper contact and taped it good. I didn't take picture so used one from radman as his work was more visible. Make sure you use a proper gauge wire so it nicely makes the contact in the plug.
Re: interior hatch pannel. I decided not to do as don't want to damage interior pannels but though about it as it is a next cheap and fast project.
I was planning to buy one more push button switch and install it next to the push button for the light on the access panel. It should take few minutes to wire to the existing switch where wires for lights are connected (additional $10 cost for the switch). Only thing that would require drilling holes into panel is to add the grip handle. You can buy one directly from Toyota for about $16. So next project were personally I would save money and do it myself and the cost would be in low $30s including shipping.
Why would you need a grip handle if you have a powered hatch? Pressing the button should close it, right?