we can only guess about Thyagaraja's intent.
You are perfectly correct. This is where intellect, reasoning, experience and objectivity has to come in. Your post takes us into the area of what may be termed musical reverse engineering. (I would even use a stronger term, musical forensic - though the crime is not necessarily negative in this instance.)
The antidote for this in our system has been the Sishya parampara, the unbroken line of disciples who would faithfully memorize and propagate the singing of their guru's compositions. Because of this, we can believe we "know" what the composers intended and how their krithis should be sung.
The key words here are: faithfully, memorise, believe, intended.
Reality is very different - in fact almost the opposite. The higher the number of quality musicians and composers who handle their guru's works, the greater the probability of it deviating from the original intent and even form & structure at times.
Veena Dhanammal & family who were renowned for preserving songs on an 'as is' condition were exceptions, which is why top artistes would flock to them, but add their own touches after learning (I am as guilty as anyone else).
I have addressed this in my book in Chapter 2, Page 7. Let me quote some excerpts:
"Indian music world did not believe in ‘locking’ compositions through precise and painstaking notations unlike their counterparts in the West. To be fair, it is no easy task to write down notations when an inspired composer is at work. More often than not, early composers or their disciples wrote down the [i]lyrics and trusted their memories when it came to melody and rhythm.
A few would document the tunes when they found time. Some scholars hold that many of Tyagaraja’s works were modified very early on by artistes like Tachur Brothers. Similarly, attempts to trace the musical thinking of Muttuswami Dikshitar and Shyama Shastri through an unbroken chain of notations often takes one through nebulous routes and in some instances, leads to dead ends."[/i]
In the absence of such an unbroken line of oral transmission, one can only try to reconstruct the intent of the composers.
Again, I have addressed this popular misconception in my book:
" In the case of super-composers like the Trinity, a number of eminent artistes have refined the works and given them a sheen which, has led to a greater awareness of their class, which in turn has triggered off exhaustive studies around their creations. But the flip side is that the probability of the primary composers’ songs being rendered as they conceived them is very low. A listener hardly knows what parts of a Tyagaraja krti, say Naa jeevadhara (Bilahari), is actually the composer’s and which segments are additions, editions or modifications by others. The exception to this are varnams and swara jatis, where outside imprints cannot be more than marginal - because they have fewer flexible parts. Therefore, it is easier to gauge the musical calibre of say Pachimiriam Adiappaiah or Pallavi Gopala Iyer through their brilliant varnams."
Ragam names might change, talam names might change, even meanings of common words and references might change, and we'd have no clue that it had happened. Unfortunately, this remains the case for many great composers, like OVK, Purandara, Ramadasu etc.,
You are perfectly correct about PD, Ramadasa and many others. However, in the case of OVK, we do
have a sishya parampara that has preserved the works even melodically for centuries. Which is why, I have been collecting as many audios as possible from this parampara over the last 25 years or so. Let me again quote from my book:
"In the case of Venkata Kavi, it is easier to assess his musicianship (or even basic intent) because of two main reasons.
(a) His compositions – till recently – were limited to a close circle of descendants (barring a couple of exceptions like Raju Shastri and Krishna Shastri). Many of these were not professional
performers or composers but they were no less passionate about preserving their ancestor's works as best as they could. Therefore they were somewhat ‘immune’ from the touches of other maestros.
(b) His compositional structure with several relatively non-flexible parts also makes it fairly deviation-proof in a broad sense. Most musicians and musicologists would be aware that it is far more difficult to structurally alter madhyamakalas and gati bhedams than other parts.
So it is as much a matter of reason, as belief in the case of OVK too. This is not to say that musical changes are non-existent in his case, it is only less probable
. A few of his compositions could have undergone various stages of metamorphosis, as much as any other composer.