I'm no metallurgist, but I don't think anyone back then could be blamed for not knowing the properties of steel at those temperatures, if that indeed was a factor. Likewise, it is hard to blame anyone for not anticipating the flaw about the water pouring from one WT compartment to another. I doubt anyone could have anticipated that many compartments would be punctured in just one accident. Colliding with an obstacle, even another ship, was a possibility. A continuous below the waterline rupture stretching fore-and-aft that far was probably never considered. It was truly a freak accident. Of course, all accidents are freak accidents the first time.

The major problem is that the crew and passengers were not drilled in evacuating the ship, so many lifeboats left only half-full. That's not bad engineering. That's irresponsible, criminally negligent management. At sea, management is called "seamanship". Also, there may not have been enough lifeboats even if they had been loaded and launched in an orderly fashion. The third class passengers never had a chance.

The mindset that the ship was unsinkable no doubt led to the loss of life. But even a truly unsinkable (if such a thing even existed) ship can still catch fire. And a severe list can make all the lifeboats on one side of the ship non-deployable.

The small rudder was not a design issue, ocean liners were designed for speed, not maneuverability. Big rudders would slow them down. In harbor, they rely on tugs to maneuver.

Another major factor was speed. For marketing reasons, the Company was determined to make a passage speed record, which meant sticking to the Great Circle route between the UK and NY. This took it too close to the ice line, near Labrador. A more southely route would have missed the ice, but added time to the trip.