It is possible, it’s just not generally supported be ISP routers. Also there is a possibility of performance issues since IPv4 NAT often relies on hardware acceleration which might not work for NAT6.

Even tough IPv6 is technically superior to IPv4 for the network operator it doesn’t have clear benefits for home users.

Having global addresses instead of NAT means less control over your LAN and these unique public addresses can track users more accurately.

No, there isn’t any more risk buying a mining card than any other used card. In both cases you should use a platform/marketplace with buyer protection options. Maybe one additional step is checking the VBIOS when testing.

The non XT is the best value of the 6600 family but depending on local pricing the 6600XT, 6650XT and even the 7600 could make sense. Just keep in mind that these are the same performance class. Some charts show the mentioned GPUs.

There are some used options e.g. 5700 XT-s are really cheap because many of them were mining card. For new cards there aren’t many options RX 6600 has relatively good value, but it’s only worth it if efficiency or features like hw video codecs are important for you.