(9.128cd) Similarly, the individual qualities cannot exist
Because you say that each of them is a composite of the three qualities.

What is referred to here is the Samkhyas say each of the three qualities of lightness, activity, and darkness are comprised of different proportions of each of these three qualities. If that is the case, then each quality does not exist individually since it is always a composite of all three. There is no point at which we can have a completely pure non-composite example of the qualities. This is their view, and Shantideva is simply pointing out the contradiction.

But we can apply the same reasoning to any sort of general principle that some philosophical school might describe. Any general principle explained would have some combination of characteristics that are in relationship with each other. If these characteristics exist inherently, then they exist independently of the other characteristics, at which point how could they ever enter into a relationship with the other characteristics? If they do not exist independently of each other, then we cannot speak of inherently existent individual characteristics of the general principle.

Prasangikas have no problem with a general principle of how the world functions. The Prasangika’s only objection is to the general principle existing inherently, in other words independent of the mind. Reality unfolds according to the laws of karma. Karma is, if you will, the general principle of Buddhists. But they do not grasp at the laws of karma as existing independently or inherently as immutable laws of nature somehow separate from our own mind.

This example shows clearly that we need to be careful when we read the refutations of the different schools do not imply a total rejection of what they have to say, but only a rejection of a certain aspect of what they say. For example, Chittamatrins say that all things are the nature of mind, and Prasangikas simply disagree that the nature of mind is inherently existent. We keep the nature of mind part but show that the mind itself is the nature of emptiness. In the same way, we do not refute the existence of a general principle, in this case karma, we simply refute an inherently existent general principle , which would be impossible.

(9.129) If the three qualities do not exist, the general principle does not exist,
In which case it is impossible to establish its manifestations such as visual forms and sounds.
And it is simply impossible for mindless things, such as clothing,
To have the same nature as feelings, such as happiness.

Shantideva’s point here is quite simple. If something does not exist, then it cannot produce anything. Since the Samkhyas say all phenomena arise from the general principle, if the general principle or its characteristics do not exist, then it cannot produce anything. It becomes another example of production from no cause, which was refuted above.

(9.130) (Other schools) “All things truly exist in the nature of their causes.”

Here other schools are saying that affects are the nature of their causes. For example the Oak tree is the nature of the acorn from which it arose.

But we have already thoroughly refuted the possibility of truly existent things.
According to you, clothing and the like arise from the general principle, which is a balanced state of pleasure and so forth;
But this cannot be the case, because we have refuted the existence of such a general principle.

The Prasangikas call this the substantial cause, but they disagree that the cause continues to exist once the effect has arisen. The acorn ceases to exist after it has transformed into the Oak tree.  The Prasangikas do not disagree there is a relationship between causes and effects, nor do they disagree that phenomena arise from causes. What they are refuting is the possibility of a truly existent general principle.