Hey Lessoc, welcome back! Let me see if I can get these answered for you.
1.) Will the Kal accept "worthy" converts as Acolytes?
So, I'm not sure I understand the question. Any human who shows potential for Ceynara's gifts can certainly become an Acolyte whether or not they hail from the Empire originally or not. However, "humans are the primary and dominant race, with the core bloodlines elevated over those that were subjugated in ancient times. Non-humans are exclusively slaves and can have no other role." As such, it is HIGHLY unlikely that Ceynara would bestow her gifts on a lesser creature. That said, "Vos-Ulin (High Slave): Gladiators, noble servants, and those that work within upper-level positions with some responsibility are of this caste. Vos-Ulin are treated relatively well and are valued by their owners. Not surprisingly, very few of the Vos-Ulin are non-human, and those that are usually have some rather extraordinary gift or reputation associated with their status." I could see a favored gladiator who has given over wholly to Ceynara and Flame MIGHT be worthy of Ceynara's gifts. The problem, however, is that elevating a lesser being to the status of Ceynara's chosen would invariably weaken the caste system and lead to greater rebellion and dissent. As such, it would have to be a SPECTACULARLY good reason for Ceynara to erode her own Empire for the sake of one individual no matter how gifted.
2.) What do builders eat and how does their agriculture work?
Builders are closest to dwarves, so you can assume they have a prefered diet closest to a very highly refined and sophisticated dwarven diet. That said, currently Builders are forced to live and work with existing cultures, so modern Builders eat whatever local cuisine is avaialble. They always eat the best, of course. As for agriculture, no Builders currently are involved in agriculture. Again, there are small numbers of Builders who work for various nations/factions. As such, they do not perform such menial tasks as farming. In that past, it can be assumed that Builders either use lesser races to do manual labor, or more likely, had a highly automoted agricultural industry. Since they can readily construct arcfire devices and golems, it must be assumed that they could easily manufactor any piece of machinery needed for something as mundane as farming. The real issue now is the severe shortage of crysarium to power their technology. It would be as if we suddenly lost all the worlds oil and gas. The vast majority of farm tractors would simiply grind to a halt. Now, we can supplament that energy, and there are emerging technologies now that might take the place, but 50 years ago, or longer, if we lost gas/oil all of our technology would be effectively useless. That is the nightmare the Builders find themselves in every day.
3.) Did builders and Faelakar/Eldakar exist at the same time on Starfall historically?
From Magic and Cosmology, "At the moment of the Shattering, Shanais and Targon infused the vast majority of their power into the continent and surrounding islands of what is now known as Shaintar. Instead of losing their power to the Shattering, their Father helped them use it to instead keep Shaintar relatively unscathed in the cataclysm. As well, large numbers of many races were transferred from other places on Starfall, preserving the diversity of sentient cultures at least for this one world. Even as the Mists settled into place and tens of thousands of startled beings struggled to understand what was happening, Shanais and Targon used the last of their remaining cosmological might to raise fve beings to a higher state – Vainar, Ceynara, Illiana, Dranak, and Zavonis." Since we know that both Vainar (Faelakar/Eldakar) and Zavonis (Aevakar) were elves at the founding of Shaintar jut after the Shattering of the Spire, AND that the Builders were a major faction fighting in the great war which led to the Shattering, the answer to your question is yes. More to the point, if this is to be understood, the Elves, and other races, appear to have been involved in the war fueld by Builder warmachines. Whether the pregenitors of the current races were active participants in the war, or merely refugees fleeing the conflict, is not stated. But, yes, the Faelakar and Eldakar existed ON Starfall BEFORE the Shattering of the Spire and, so too, did the Builders.
I hope this all helps. Let me know if I can answer any more questions, or clarify and points here!