How do I mount this Topeak rack onto this road bike with disc brakes?
You have the seat stay mounts on the frame (those are the two thin tubes that go from the rear wheel drop outs to the top of the seat tube). What you need is mounts on the rear wheel drop outs. Typically, they are part of the dropout and/or frame. In your case, you have none.

That being said, the P-Clips and Tubus Clamps will not be useful.

The Tubus Adapter Set for Quick Release Axle Mounting should solve your problem. It is built for a Tubus rack which has two mounting holes at the bottom that are meant to be used simultaneously, thus the two slots. You can choose which one is appropriate once you loosely install the rack and check how level the top is. Installation instructions and a drawing to go with it can be found here.

I should say I have not done this actual installation myself, but I see no obvious reason why this would affect anything else. There are comments from people that have used this with discs and there's a spacer to keep the mounting plate away from the frame, so you should be good.

The only concern I have is there seems to be nothing to prohibit the mounting plate from rotating fore or aft. You may want to invest in a couple lock washers and a proportionally longer quick release.