Okay, so after some time, I think I've almost completely figured out how spawn works after many observations. Out of everything I've gathered, here's what I've got.
Spawners Are All Individualized: This is most apparent when looking at Great Frog and Night Stalker spawns. Great Frogs have four spawners that take longer to produce a mob than the rest of the Great Frog spawners. All the spawners that take a longer time are at the Yard Sale itself.
Spawners Have a Set Spawn Rate: Spawn rate is constant and is based on the spawner, not the mobs. The internal clocks of spawners wait a certain amount of time before producing another mob. Mobs only spawned instantaneously because players kill a mob just before the spawn clock hits its mark. Spawn rate defaults to 0 seconds when one isn't set, as noted the first time Ground Spitters made an appearance in the game.
Spawners Start Spawning at Random Times in a Fresh Room: While it is a bit difficult to test the theory now, it is supported by the multiple Batling spawners put on the arches in the 2nd cave area, which all spawned Batlings at seemingly random times despite the set spawn rates.
Areas and Spawners Require a Back-End to Spawn Monsters: This was discovered when I reported a bug where monsters wouldn't spawn in the campsite of Teplar Woods. It seems the lack of spawns was the result of monsters not being set to spawn in the area.
(Debatable) Mob Spawners Have a Set Spawn Cap: There are hints that spawn caps can be individualized thanks to the Night Stalker cage and the dreaded Tesla Crawler+Policeman rain, but so far, it seems that mobs have a set spawn cap. Rafflesias in the first Red Leaf Hills area and Claystones cap out at two monsters. Grunts had capped at five monsters in the first cave area, but there is a possibility that this was upped to 7 as I've observed later on. Spawn cap seems to be determined by monster since Shagdaws, Petunias, and Hellshags weren't affected by the Spiked Squisher's spawn cap.
Here's what I am currently unsure about.:
Multi-Spawns: This feature was introduced with Tesla Crawlers and is present in Tesla Crawler and some Policeman spawners. It seems a multi-spawn counter is implemented; however, I'm not fully sure how multi-spawns work.
This was all for natural spawners; artificial spawners such as the Leafling button and the Singing Trees are excluded since they seem to play by their own rules. But yeah, there's what I've gathered.