It's hard to talk about the advanced phases of Buddha's original path in the Canon, because no one can say with certainty that he or she understands exactly what those brief and cryptic Canon descriptions mean. Many people claim they "kind of" understand and have reproduced these phases, but their explanations don't match, and we don't have the Buddha available to confirm who of them got it right.
Luckily we know enough about Buddhism's underlying principles to recover the overall system, and this can help us figure out the meaning of these obscure passages.
First of all, there's no doubt for me that Buddhist liberation (whether we call it the end of suffering, or just Liberation in a broader sense) is about transcending the limits of mental constructs. It's clear to me that dukkha arises as a clash or irresolvable contradiction within a stream of experience. The clash, in its turn arises from mental constructs contradicting each other or contradicting reality. In a way, all mental constructs are simplifications, so as long as you attach to them as something complete and valid, you are prone to suffering.
This gives us enough context to understand Buddha's Noble Truths and his "liberation through wisdom" in principle. Once you clearly understand how exactly suffering arises and see the issue with attachment to mental constructs, you can learn to stop grasping, getting immune to suffering and free from all limits.
Another thing we can clearly see from the suttas is that most of the time Buddha taught a gradual, incremental method of implementation of these principles. First, one has to remove the coarse suffering by removing the coarse causes of coarse suffering. Then go to the harder to see and more subtle but also more fundamental types of suffering. There are many steps on this ladder.
All of the above is pretty clear but it is rather high-level understanding in broad strokes. What gets more difficult is to understand the step by step instructions given in the Canon. There are several different progressions given in the suttas, describing alternative ways to proceed.
One major variation of the path goes through the Four Jhanas. As I explained many times on this site, the Four Jhanas do not talk about the content of mind as a kind of fixed state, but rather about one's attitude to experience on each of the four stages. The meditator learns to overcome mind's tendency to create suffering by setting expectations and comparing them with reality. The process goes through several stages of refinement, until the meditator scrutinizes one's attitude to this meditation itself - one's motives and expectations - culminating in the direct non-intellectual experience of Tathata. This is known as the desireless door to Nirvana.
Another variant we see in the suttas is the one that goes through the Formless Ayatanas (never called "jhanas" in the Canon!) and the signless absorption. This one is also an incremental progression from coarse to subtle, but instead of reflecting on one's attitude and expectations, the meditator engages in the direct non-intellectual study of mental constructs and the workings of subject-object mind. The idea is to get a first-hand look at how all constructs are relative and how subject and object are two sides of the same coin. To do this, one scrutinizes one's mental experience of objects delineated against a series of ever-expanding backgrounds. First, spatial objects against the background of "space", then, the mental experience of "space" against the background of one's "consciousness", then reflect at one's own "consciousness" taken as an experience against the background of "nothing". (The key insight to gain from this practice is how each of these consecutive backgrounds serves as the subject observing the previous object, this subject itself becoming an object of an even deeper subject on the next step and so on.)
Then there is a stage that I don't fully understand - the Neither-Perception-Nor-Non-Perception. I don't think anyone really knows what that means. We know it must be more refined than the previous one, so it's probably some kind of background against which the experience of "nothing" is delineated and recognized by the mind.
What I do understand is the next stage: at that point meditator realizes that even recognizing "nothing" is an act of semiosis and finally arrives at the mind that does not perform any semiosis whatsoever -- not grasping at and not interpreting any signs. This is the signless absorption.
And then, as Buddha always says, one realizes that even though the signless absorption is absolutely free from suffering, sitting in this sterile bubble is not something that can last for infinity, because it is a conditioned situation that sooner or later ends. So the value of the signless absorption is not in it being one's final destination, but in giving the meditator direct non-intellectual understanding of how our experience of world and the self is just a bunch of mental constructs and therefore should not be taken too seriously. This is known as the signless door to Nirvana.
If you take a 40-thousand-feet-high look at both of the doors, you can see that the common theme there, in both cases the meditator does not stay within a sterile state of no expectations/desires as in the Fourth Jhana or the still mind of the signless absorption. Instead, both of these doors lead to the same breakthrough to the unconditioned non-abiding Tathata, the perfect attainment of the Third Noble Truth right in the middle of samsara.
Now, when we look at the Bahiya Sutta, we can assume that the Buddha is most likely talking about the very end of the path. Bahiya is presented as an advanced student who had enough reasons to think he could have been an arahant. Unlike in the Cūḷa Suññata Sutta, with Bahiya the Buddha skips all the gradual teaching and gets right to the heart of the matter. Finally, at the end of the sutta Buddha declares that Bahiya has attained the final Nirvana. Clearly, he was a very advanced student ready to receive the most advanced teaching.
To me, the way Buddha talks about the seen, the heard, the cognized etc. being just what it is, looks like he talks about Tathata - the experience of an arahant.
Could he be talking about the signless absorption? I don't think so, first and foremost because in silent absorption one does not delineate the Ayatanas of the seen, the heard, or the cognized. The signless absorption is even beyond the Ayatana of Infinite Nothing and the one that goes after it. But here we are talking about Seen, Heard, Cognized as separate modalities. Clearly this is not the signless absorption.
Another useful detail Buddha adds is "you will not be 'with that' ... you will not be 'in that'". This reminds me of Buddha's teaching about anatta and how we should abandon habitual thinking "I am that; I am with that; I am in that" and so on - regarding any and all phenomena.
Finally, the Buddha concludes by equating this attainment with the end of suffering, the very Goal of Dharma. Is the signless absorption the Goal? No, the Buddha himself said it is not.
Instead, Buddha says, seeing what is seen, cognizing what is cognized without grasping at mental constructs, "you will be neither here nor beyond nor in between the two". In other words, neither in samsara ("here"), nor in the sterile bubble of samadhi ("beyond"), but in the groundless non-abiding nirvana.
So I think what Buddha describes there is quite simply the perspective of an arahant, not any intermediate stage preceding arahantship. Arahant looks at things with a dispassionate mind, not grasping at signs, seeing things yata-bhuta (as they are) without adding his or her expectations, attitude, or indeed anything related to self.
Based on all of the above, I think the answer to your question is: yes, the teaching in Bahiya Sutta logically goes after the teaching in Cūḷa Suññata Sutta.
Remember how I said the Four Jhanas is the desireless door to Nirvana and the Shunyata is the signless door? These are two alternative paths and the Bahiya Sutta describes where you get after walking either of them to the end.
So, yes, Bahiya must have either realized shunyata or have attained all four jhanas before meeting the Buddha, otherwise he would not be able to understand and perfectly implement the final instructions. Too bad he was stupid enough to get killed by a cow, otherwise he could have grown to be the second Shariputra, starting his own school of Buddhism, who knows ;)
