I simply can tell you again what you've already heard from me and a lot of others in this message board: Ships are built to earn profit for their companies. And when they have reached an age when they are not longer reliable, not longer in the paying public's taste, they have to be put out of service. And so sad it is - but this mostly means scrapping.
About your questions:
Why did the torpedo hit the Lusitania right in the boiler room?
I'm not sure I got the point, but simply because it had to hit somewhere if it didn't miss. Pure accident.
Why did the Lusitania sink in 18 minutes?
There was an enormous second explosion following the torpedo hit which damaged the hull badly and destroyed the whole structure of the bow section. This caused intense flooding within only a few minutes.
Furthermore, there were additional reasons for the fast sinking:
[*]A lot of the waterthight doors could not be closed as a result of a quick failure in the ship's electric systems and the damage of the bulkheads follwoing the explosion.
[*]The engines were still running and could not be stopped. Lucy was pushing herself under water.
[*]It was a bright, sunny day. A lot of portholes were open and caused additional flooding - especially as there was a great list to starboard.
What is Lusitania's state now?
Quite bad. She's resting on her starboard side, the hull broken, the upper sections slipped off her, the funnels and masts are gone.
What would if Lusitania survive the war?
Well, she would have been converted to oil furnaces as the Maury, had served well and - sorry - would most likely have been scrapped in the mid 1930ies.
About scrapping: What do you want to hear? We don't live in a romantic paradise, but in a world in which money has to be earned and people have to find jobs. Think about ship companies losing profit because of operating old and no longer adequate ships - they would fail and had to fire all their workers. Steelworkers have to find work, and ship's steel is one of the most important basics for steel economy even today. Perhaps it helps if you imagine that scrapping is necessary for saving peolpes' jobs. And sorry - but even as a great shiplover I say: To get people employed is much more than preserving an old ship.