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Yeah I don't understand why these plants weren't built on the West coast, and why they weren't built on higher ground to mitigate against tsunamis.

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You can't build reactors on high ground because they need a very high coolant flow. The (elderly) Fukushima BWRs pumped 480Mw into the grid while they were running -- but that's at the far end of a lossy thermodynamic chain (steam is used to spin turbines which drive dynamos); the thermal power output is probably something like three times that. So you need a heat sink into which you can continuously dump about 1.5Gw of heat. Water fits the bill, but you need a lot of it -- either a big river, or an ocean.

Which is why civil nuclear reactors are almost always built close to the coastline.

Is it feasible/economical to have a secondary power plant to convert all that heat into electricity?

Not unless you repeal the second law of thermodynamics -- the 1.5Gw of heat was already used to generate 0.5Gw of electricity. In general, no thermal power plant can convert much more than about 40% of the input heat into useful work.