I rented with Toyota and others. Paid for the "menseki hosho", "免責補償" , and wrote "I decline CDW" next to my signature.

That means NOC will be covered by credit card. Liability deductible is covered by 免責補償.

For Toyota, 免責補償 reduces your LDW(and similar) deductible to zero, reduces 3rd party deductible to zero.

If I don't use credit card insurance, I need to buy 免責補償, but I still have to pay NOC if things happen. Some rental companies can have NOC waived if you pay extra, but not Toyota.

If I use credit card insurance and don't buy 免責補償, everything is covered except 3rd party deductible.

>> But no idea if I'm playing with fire in doing so

Many foreigners don't buy 免責補償. It is a very expensive "insurance" if you rent for 15 days or longer, for example.

2000 X 15 = 30k premium, for that you get covered for the deductible of 50k CDW + 50K liability. That's a 30% premium rate for 15 days.

I usually rent only for a couple days, so I pay it. Amount offset by the Hertz discount.

>> Google Maps works very well in Japan,

Until it directs you to a business that is permanently closed. Also the routings are not optimal compared to Japan based in-car or online GPS.