Ultimately, regardless of feasibility, it's simply a solution which they didn't even consider, because again, as you recognized why would they. That's politically suicide even if it's deemed ecologically sound plan. Which circles back to your first point, does any country dump tritium in their local lake? Not publically, and Fukushima is notoriously public politics. But you know what most try to do with nuclear waste? They attempt to engineer supremely expensive local storage solutions, and frequently fail only to spend stupid resources to warehouses it domestically, instead of dumping that into the commons. It's 60 olympic swimming pools worth of water. It's trivial storage problem for nation state. Hence problem 1/2/3 isn't even a problem especially if they bothered with local dumping studies which again, is 3 tankers per day for 30 years. It's nothing. Desalinating 1.25m tons is also trivial, and not even that costly. A 10m gallon / day plant cost less than 100M and sort filter that in a month. Which is peanuts versus aggregate clean up cost, and even potential lost fishing revenue. Cost isn't driving this decision to release, politics is. And it's not regional geopolitics because JP isn't releasing this water out of spite for her neighbours. Let's not forget, it's not PRC but most of region as well as countries across the Pacific not pleased with this decision.
Which ultimately circles back to point 4, and my broader point. LOCALs don't like it. LOCALs vote. Can't risk LOCAL votes even if they're irrational, and EVEN worse if their rational distrust of TEPCO becomes reality down the line. IIRC even more containated water already leeched into ground water. Risk of future leaks, however "safe" this water is, and heads will roll. So the politically expedient solution is to dump into ocean and remove any potential risk. The political math is simple, they would never even consider local disposal or long term local storage due to the risks, so into the ocean it goes. Which again, is fine. Fine in the sense that it's a calculated decision. But let's just acknowledge that's whats actually driving decision making. Less science and more domestic JP politics. They didn't have to dump this water. A 5T economy can afford to store it indefintely and ignore geopolitical shit show. But domestic JP politics want that water gone from JP territory, so into the ocean it goes.