So right around the time you posted your questions, I left for work. I gotta say, I kinda like the enthusiasm you had in your first reply. Always happy to help if people are willing to learn!
1) The other guy who answered is right. You build hybrid, with the resists to deal with the fed champion. So if it's the APC who's fed, you build a Banshee's Veil or Spirit Visage- has the resists to deal with the AP, but some health so you're not screwed if the AD champs come after you- or Randuins if the enemy has 3 AP, but only the ADC is fed.
2) In solo-queue, you can't really rely on your team to do damage. If you're fed enough that building damage would let you 100-0 the enemy, by all means build damage. If your role in a team-comp is to be the tank, you can always transition into a bruiser later, building 2 defensive items (generally Randuins and Banshee's Veil) leaving room for boots and 3 offensive items. The idea being, since you're fed, the enemy's behind and still won't be able to kill you. You build damage and work to get your allies fed, then once they can also dish out the pain you start building tanky and become the tank they need you to be.
3) If you're gonna get Warmogs, get it early. Like I said in my first comment, as you transition into mid-game, you want to load up on health so you can be tanky against both damage types. Especially if you're the jungler, you'll be roaming and fighting every lane, so you don't want to have, say, a Warden's Mail as your only tank item when you're ganking Leblanc, or a Negatron Cloak when you're ganking Zed. On the other hand, if you're laning, your first defensive item should be suited to your lane opponent.
Personally, I'm a big fan of getting a component item to face my laner, then keeping that as just a part while I build health. So if I'm a melee facing Lissandra top, I'll get a Spectre's Cowl (the passive that gives you increased healing when you take damage makes laning soooo much easier) and then I start building health- maybe Rod of Ages if I'm Maokai, or Warmogs if I'm Mundo. The early resists help my laning against the Lissandra, but as I transition out of laning phase I need to be tanky against more than just my own laner. So I'll finish my big health item before I touch the Spectre's Cowl and turn it into Banshee's or Spirit Visage.
On the other hand, if I'm Nasus against Riven, starting Cloth-5 and rushing an early Glacial Shroud will significantly help my survivability in a bad matchup. Then I can hold on to the Glacial Shroud and start picking up some health before I turn that Glacial Shroud into a Frozen Heart.