You are not at liberaty to invent your own Dharma - well you are, just don't call it Candrakirti's intent.
False, faulty, incorrect, etc. All of these apply to relative truth.
Khenpo Karl Brunnholzl often use a alternative term "the deceiving" which I also like because doesn't suggest useless: [i]"Generally speaking, if a given philosophical system differentiates the two levels of seeming and ultimate reality, then in whatever way it does so, one it speaks about seeming, relative, or deceiving phenomena", it must also accept this mean that such phenomena are precisely something that is not established.
Ok, you are not understanding something -- kiun rdzob is relative truth, but actually means "totally obscuring" in Tibetan.
But false perception is mthong brdzun, so what Candrakirti is clearly saying is that false/faulty/incorrect perception is relative, or totally obscuring, truth.
The two truths are about how objects are perceived. They can be perceived in only two ways, correctly and incorrectly. Perceiving them incorrectly, a false perception of them is called relative truth. The word brdzun pa means "to lie" as well. Further, for example, there are two schools in Yogacara rnam bden pa and rnam brdzun pa i.e. true aspect and false aspect. The latter is the higher of the two. The term brdzun pa means false.
So a false perception is relative truth.
When Shantideva is taking about the two truths, he says - ultimate truth is beyond the mind, because the mind itself is relative. The mind can never apprehend ultimate truth.