I never truly understood how the Las Plagas came back from fossilization. I remember reading how they invaded the miners underneath the castle through an airborne substance... which still makes no sense to me to this day. Their eggs also confuse me because they are never explained fully, at least not in game.

Saddler also had the biggest parasite so does that mean he laid the eggs? Where the hell did he get infected? What role did these parasites play in the prehistoric era? Did they infect caveman to survive? Why are they underground? Where they put here by aliens?

Im going off but Las Plagas always stood out like a sore thumb in the series.

They never really "came back." They were sealed by the first Salazar castellan quite literally beneath the castle and basically left to die. However, they instead went into a dormant hibernation state and their physical bodies became fossilized over time. Then, when Salazar released them, their genetic material was extracted from the fossils and cloned (hello Jurassic Plaga!).

However the fossils themselves released spores which, when inhaled, matured into adult Plagas. Essentially, the spore method is a substitute for the egg method. All Plagas are able to lay eggs in a new host directly. The "ritual" Ashley is subjected to was a (still unknown) method to give her human body the ability to implant eggs into other people (Salazar was also capable of this through his fingers).

The "dominant species Plaga" which Saddler, Salazar, Mendez, Krauser and Irving were infected with was created through repeated genetic engineering with the regular Plaga. It had greater mutagenic properties and allowed the host to completely control the parasite (and other subordinate Plagas). Incidentally, it's hinted that this dominant species was created through the use of the t-Virus.

I had a feeling this was going along the lines of Jurassic Park, only thing they are missing is T-Rex Simmons..... and wtf Salazar had egg laying fingers!? That guy was more fked up than I realized.

Was there any more information on the Plaga before they got sealed away? Or is that left in mystery?

Nothing aside from the fact that Los Illuminados worshiped them, they were indigenous to that mountain region and the Salazar family knew how to control them with sound. Since they're a natural creature there's not much opportunity to give them any special history.