@satellite ktavatu

Are /ktavatu words always used as the main word of the sentence, taking first ending by default?

Not always. Just most often.

The base uktavat- उक्तवत् will most often be main word in its sentence and will linkto to its doer, both doer and uktavat- उक्तवत् will have first ending then, and uktavat- उक्तवत् will translate as "said" —

Rtam uktavAn saH ऋतमुक्तवान्सः "he said the truth"

Rtam uktavatI sA ऋतमुक्तवती सा "she said the truth"

However, like nearly all other words that show the doer, they can be satellites of a word that is in another sentence, if that word shows their doer. So uktavat- उक्तवत्, when it is main word, means "said" and has first case,. But when it is a satellite it will mean "who had said" and will have the same ending as its planet. So we can make uktavat- उक्तवत् linkto the second-ender rAmam रामम् in this sentence —

sItA rAmam aikSata vismitA
सीता राममैक्षत विस्मिता
"sItA सीता, all astonished, looked at rAma राम"

and then we get

uktavAntaM RtaM sItA rAmam aikSata vismitA |
उक्तवान्तं ऋतं सीता राममैक्षत विस्मिता ।
"sItA सीता, all astonished, looked at rAma राम, who had said the truth."