Dear Sri around 1320 when malik and tha khilgis had ran thro tamilnadu the only prevailing power here was the cheras headed by ravivarman kulasekaran. an ambitious king he laid stake to the pandyan throne also, on the basis that pandyas were his maternal grandparents-- in fact he crowned himself as the pandyan monarch on the banks of river vegavati.
I think the dissassociation began after that. there were no cholas to speak of. the pandyas had moved further south to tenkasi. madurai was with the muslims. so i think the cheras decided to stay in the hills perhaps when the naiks came they could not co ordinate with them. but the active chera legend is that there were 3 men one a nayanmar, one an alwar and one -an early convert to islam and emigrant to mecca all in the royal family of kerala.but this was all in 8th century ad . venketesh
But coming back to Chera history, I think we have to look at origins of monarchy itself in Tamil country, where the tribal/rural chiefs like Ai, Velir etc., were ruling the country.
I think the earliest rulers of present kerala region would have been such chieftains probably Ai.
Pre-history - Jumping to the aryan expansion theory, we can see many rishis being credited with the exploration/colonisation of new regions, for eg., agastya. So the myth that parasurama created the kerala country might have some truth related to such explorations, also their is a possibility of sea waters receding in a natural phenomenon during the same age. Often myth has some underlying truths. perhaps mytho-geologist should check whether marine fossils are present in W.ghats.
That the Mu-vendar came to be be by slowly gathering authority with the support of such chieftains or supressing them makes political evolutionary sense.
I think Kerala per se must have become a Chera country during the Kalabhra rule, were rulers of Karur were pushed out of tamil country proper.
So that by Sangam age, we have Seralaadan and Senguttavan in that area.
But evidences of Cheras seem sparse compared to Chozhas and Pandiyas. Sthanuravi of Vijayalaya/Adithya times. Kulasekhara the Azhvar. Cheraman Perumal. The ambiguous chera who went to Mecca. couple of Bhaskararavi varmans and thiruvadis. And finally ofcourse the one who fought against Muslim invaders.