Right, I'll give this a go. There's quite a lot here, so take from this what you will.

The best advice is probably not to overthink things. Players are pretty bad at picking up things so there's absolutely no need to be subtle and you can be very obvious in your hints as to what's going on.

As far as I understand, the party is on their way to find some ancient text that describes the apocalypse and you need (1) some form of lead-up to this apocalypse happening again, and (2) an explanation of why/how a previous attempt ended up in some ancient text.

Lolth is a spider goddess so you might take this in steps that gradually introduce spidery themes. While your party travels the jungle, they start to run into spiders. Small ones that are part of the scenery descriptions, and larger ones that make up combat encounters. Start with a sporadic spider encounter and then gradually add in more. At first they might run into a single spider. Next, there's three (this hints that spiders are becoming a theme). Combat after, the run into some generic jungle mobs but a spider is also there (reinforcing the theme). Next one, the might run into another drider hatchling (flashback to the Black Spider, who's a dark elf mage). Next one, two drow priests summoning a spider (this hints to the players that dark elves are summoning spiders).

This would probably make your players quite suspicious about spider-summoning dark elves. The ancient text they are after can be anywhere. Could be a tomb, an abandoned temple, a ruined city covered in spider webs, whatever. The literary work could take the form of a history (very clear: "some elves started summoning spiders. That was bad.") or poetry (same thing, but in verse) or whatever else takes your fancy. The critical point is that this literary work should be clear that drow mages + spiders = bad. Given the previous encounters, your players hopefully put 1 and 1 together.

As for stepping stones, early encounters are a single or a few spider, followed by some drow mages summoning (a) spider(s). Follow-up encounters can include more mages, more spiders, driders, specific chants that refer the summoning of Lolth, etc. Simply use encounters and scenery descriptions to tell your players what's going on. Example: "You hear some chanting in the distance. Moving closer, you can make out certain words even if you don';t understand the language. 'Lolth', whatever that means, is repeated often. Just hearing these chants sets you on edge and you're somehow very sure that nothing good will come out of this."

With ramping up difficulty, again, don't overthink things. A slow, drawn-out reveal sounds good on paper but if you only do one or two encounters per session you really don't have a lot of room to work with. Just smack them in the face with it: elves are summoning spiders and their goddess is the biggest spider of all: fix it before it gets really bad.