1) Yes, that's normal temps for the stock Intel HSF on a 100% loaded CPU. They're not designed for sustained heavy workloads like F@H.
2) 80-90C is getting toward the upper limits of what the chip was designed to handle, but they will throttle themselves to prevent damage if needed. Technically I think it'd be fine, but 65-70C max would be a more comfortable temp. Everyone knows that cooler component temps encourages longer lifespans, but I don't think you'd be able to find any data out there to prove what the tipping point is for a given part. Law of diminishing returns applies here.
3) I'm not sure how many people have actually listened to a stock Intel HSF lately and how many are just making generalizations, but they're not terribly loud even when the CPU is at 90C and haven't been for quite some time (I just had a Haswell one going Prime95 on an open test bench a couple months ago). A 120mm aftermarket HSF would definitely keep the CPU cooler at the same or slightly less noise though. Stock AMD heatsinks still suck as much as they used to, P4 HSFs were bad, but IMO C2D/C2Q was a major improvement, and stock Intel HSF's have been decent at stock clocks and "typical" workloads since Sandy.
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod