Hi Akvan,
Thanks for your explanation! You are able to explain things clearly in a simple manner, like how you explained about why a Buddha does not attain any stages of magga phala way earlier before Enlightenment to y not in another post.
This is what I think: I agree with you that to get out of this cycle, we have to eradicate avijja completely and without a trace. In order to do so, we cannot just focus on avijja, even though it is the formal starting point in paticca samuppada. PS is actually a cycle with no strict beginning or ending point, where each link is strengthened by other multiple links in forward or reverse order.
We must also focus on reducing tanha (which is related to asavas and anusayas) as well. In order to weaken one of the pair (avijja and tanha) significantly, we need to focus on weakening both simultaneously.
This is where the Noble Eightfold Path and Mundane Eightfold Path come in. Right action, right speech, right livelihood, right effort, right concentration, right view, right thought and right mindfulness on the Mundane Path help us to control our asavas and anusayas to an extent, preventing or at least reducing their further strengthening, and to a much lesser extent weakening them. It is only on the Noble Eightfold Path, however, that they start to weaken significantly. Anapana sati and sathipatthana are also vitally important in subduing anusayas and asavas.
I think Lal has said that both avijja and tanha are needed to create suddhastaska. Hence the pair of avijja and tanha are deeply intertwined and (almost) inseparable. Looking at patticca samuppada, with the other links of sankhara,etc., I think both avijja and tanha also create or give rise to rupa below the suddhastaska level, i.e. sankhara, vinnana, vedana, etc.
In patticca samuppada, avijja paccaya tanha through the intervening sankhara, vinnana, namarupa, salayatana, phassa, and vedana. Yet tanha can also paccaya avijja in the reverse order, I think.
Of the two, a much more significant breakthrough would occur when avijja is dispelled, rather than trying to control tanha directly. However tanha must also be controlled to a certain degree via sila, or the mind would be agitated and would not be able to cultivate panna to dispel avijja.