Word Sangam first appears in Jain Vajrandi Tamil Sangam established around 4t-5th century.
Clearly Growth of Tamil has been associated with Madurai. We have traces of that in Purananuru.
Surely some kind of scholarly collage existed in Madurai. But to give description that Nakeerar headed and all poems had to be ratified by the madurai sangam is later legend.
And also in appars version no episode of nakkirar netrikan and por thamarai kulam.
Only Thiruviyaldal puranams its get a rebirth.
The most probbale reason to consider Nakeerar to be head of Sangam was that
there are atleast two poets named nakeerer... one in Sangam era composed nedunalvadai and other small poems.
Another one in and around 5-9th century who wrote a commentry to Irayanar Aga porul. Irayanar agaporul has a legend it was written by god himself and presented in a forum of Scholars. In this commentry only Nakeer says about first three sangam(first ever reference).
So later commentators assumed that sangam poet and later poet were same and some kind of forum existed for ratification of Tamil works. Mixing up of these two poets made later commentators assue that Nakeerar should have held the top post of Tamil Sangam.
How many madurai....have heard of 2 earlier ones which were taken by sea b4 the pandya shifted it further inland. Will leave vairam to fill in the rest
Possible sea erosion or The capital was too close to Lanka and Vulnerable to their attacks.
But Surely it looks like the have moved Inland from being a coastal capital.
Pandiyas were always know for their coastal trade and pearl fishery.
Similar situation can be seen in Cholas
Cholas had two capitals similar to present day S.Africa(has three capitals). Uriyar the inland capital and Puhar the costal capital. Puhar port became un usable due to silt formation and hence went out of favour. After that Chola capital was permanently based only in Inland Uraiyur Tanjore GKC.