A few noob Delve questions

Hi guys. I started PoE near the end of Ritual league and almost completely ignored Delve. This time my necro seems capable of progressing a bit (i'm nowhere near "deep" delving but moving towards level 100 which is a big deal for me :p ) so i'm looking at what i can gain there and a have a few questions.

1) Is there a better area to search for Abyssal city for the 6 sockets recipe? Is it more frequent in abyssal depth biome or totally random? At which depths can i encounter it? Should i move away from the central shaft to encounter rare biomes? (i've seen a vaal outpost during ritual even though i didn't get very deep at all, but still no rare biome this time)
2) Same for the special biome where i have a chance to find the 6-link.
3) Where should i look for fossils? Am i still not deep enough? So far i've only found one fossils node, unconnected (had to blow a wall).
4) Not reward related but is there some mob that puts you into darkness? Sometimes while delving my screen turns black, then i can see the purple effect from delve darkness. So far it never lasted long enough to threaten my survival but this occurs while i'm close to the crawler.

1. Abyssal city is it's own biome and can spawn anywhere past depth 52. Keep going down and look for the golden border cities have. Going sideways doesn't give more special nodes/biomes, depth is the only factor

2. Minimum depth for Primeval ruins is 151. Can see more detailed info here

3. Most fossils are behind destroyable walls. Fossils behind walls start being common around depth 150. Deeper - better.
There are some rare fossils that only drop from special fossil nodes (can see under Biome specific nodes here). Smuggler's stash fossil node is one worth skipping since it only gives few common fossils. For common fossils walls are the best

4. Big blue lion looking thing can do EMP blast and disable your cart for a few seconds. Like you said it is rarely threatening unless you are already in trouble. Can just use a flare to get the light if you need

Thanks for the detailed info and very useful link (i knew the wiki but not the other site).
So fossils need to be hunted behind walls rather than nodes. OK, i was doing it wrong then.

Found my 6L recipe at depth 182.

I went to approx. 200 and then sideways with two paths in parallel to cover a bigger area.

Depth 100 is fine for fossils, but as mentioned you need to go into all the dark pockets along the path and break all of the walls. Learn to tell when paths have walls between nodes and how many walls there are on that path. Basically the mine is a grid and if the mine cart passes an entire grid without a stopping point there will be one wall in that square. If the path is long as passes through three grid spaces that path will have three walls and so on.