I had no clue about the story, really, when I played the game. I wrote the Diaries quite a bit after the fact in most cases, and it was then that I thought "maybe I should check to make sure if anything I'm saying is right." That's really when things opened up for me. In a way, finding out some of the realities after the game was over was part of the experience for me, just like the wikis.

Darkroot is huge. To this day I'm not 100% sure where I am in it at any given time. The key boss there, Simple, is Great Grey Wolf Sif, who plays an important role in the story and can really be approached at any time given your current level. I forgot about her until much later, so it was an easy fight, but your mileage may vary.

As to finishing the game in the foreseeable future... this is the only game I'm aware of where one half is considerably bigger than the other half. If you're fighting Ornstein & Smough, you're at - structurally - the halfway point almost exactly. You're also at the third of those psychological barriers I mentioned. Get past it and you'll know beyond a shadow of a doubt that you can finish the game if you choose.

Interestingly, despite being the halfway point, the second half goes faster. Something changes in your mind.

Simple, it will be interesting to see how your experience fits in with my authoritative, chosen path :). I never even came close to defeating S&O by myself, and I think your greatest danger is leveling your self out of range of people who can help. Remember that the acceptable range of summoning partners is +/- 10 levels and I would have been 60 something when I was there. If you get too far out of the range of people who are likely hanging around Anor Londo, you put yourself at risk of not being able to find someone to summons. 

While trying to do it single player only may be what you desire, and plenty of others have done so, I found S&O to be easily the hardest encounter in the whole game. The downside of taking on humanity and looking for help is, of course, invasions, which it seemed to me to be most prevalent around that area. 

How'd you get to the Depths area without having the basement key from the tower in the moonlight butterfly area? 

Congratulations, Helmut! If it makes you feel better, Dark Lord is the good ending.

Choices have no impact on gameplay, it's true, but it's worth pointing out that you only see certain things if you make (or avoid) certain choices. Indeed, as I alluded to in the Diaries, there is a path that, if followed, would significantly alter the player's sense of motivation and who's on which side. Said path is never described and never put forth as an alternative. The game never tells you how to trigger it, and you're never told if you missed it. It includes an important character who is otherwise never discussed and removes another who is otherwise an important cornerstone.

I was extremely drunk when I defeated O&S the first time and have little memory of the event. Sometimes while intoxicated I'm able to enter an altered state where my gaming prowess is greatly improved, to near-superhuman levels. Most times I just fall in holes a lot. But from what I've seen online, a combination of crowd control and judicious use of those pillars as obstacles are key to your success. It's really helpful to have a third in there with you, if you happen across a summon sign. It seems to disrupt them quite a bit, having to focus on three instead of two or one.

My meta-game was staggering around desperately trying not to die, pounding estus like a 20 year old on Mikonos, while my summoned partner did the heavy lifting. 

I am pleased with the way my character worked out. I finished at level 90 or so, and was at 40 dex, with 25 int, and the rest into endurance and vitality. I dabbled in the magics and got nicely along with the homing soulmass delivering 200 damage which is a really nice thing to have going into a battle. I especially like the way you can target a quickly closing foe with the Soul arrow when they are stunned by the first soulmass attack. 

Sadly, while I was dabbling in NG+ my level 100 character fell off a platform, the game crashed, and the savegame is unloadable. Goodbye little Bubba-G-Ho-Tep, I'll miss you. @Simple, if I were you, I  would continue to create your own savegames as a rational precaution.