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Building a nuclear power plant on the coastline of a subduction zone in a country with a long history of earthquakes and tsunamis does seem like a questionable idea.

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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.

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.