Is this a bug in that Unleashed version which might be fixed if I upgrade the R510 to 200.8.*? Can this even be done while there's another Unleashed instance on the same subnet? I know the R500 can't be upgraded to 200.8, so it'll have to remain on 200.7 or switch to standalone firmware.ipv6 is not supported in Unleashed so even if this bug is valid, it won't be classified as Unleashed bug. Unleashed shares the AP library with SmartZone so new Unleashed release would use the newer AP library from SmartZone, and thus if this ipv6 bug does exists, there is a good chance it would be fixed in SmartZone and thus Unleashed benefits from it as well. Unleashed does not support multi-tenant so if you upgrade to 200.8 or 200.9 (newly released yesterday), you would have to do the whole network. In your case, the R500 cannot be upgraded to 200.8 onward and thus you are right either stay on 200.7 or standalone, or you can consider Ruckus Cloud which is far better than run it just standalone.
If I switch the R510 to standalone firmware, is there reason to believe the problem will remain, or is it potentially a chipset/hardware limitation on the R510?ipv6 is fully supported on SmartZone with R510 so no reason to believe hardware limitation. Worth try the latest standalone firmware to rule this out.
Are there any other configuration tricks to try which might let the R510 work with IPv6 on Unleashed 200.7? I know that IPv6 is unsupported on Unleashed, but the documentation is very vague about what this means, and having had it actually passing IPv6 traffic correctly on the R500, I was very disappointed to find out that it didn't work at all on the R510.ipv6 is not supported in Unleashed. Rather than try standalone, you can isolate your R510 and flash with the latest 200.9 Unleashed to it, then try the ipv6 test. But do mix your other APs on the network otherwise they will be updated to 200.9 as well, which obviously those older APs (eg: R500) will not be supported. You can get R510 200.9 here:
PING6(56=40+8+8 bytes) fe80::1401:54d7:ba32:14b5%en0 --> fe80::14c6:97c1:7a3e:b28f
16 bytes from fe80::14c6:97c1:7a3e:b28f%en0, icmp_seq=0 hlim=64 time=218.095 ms
16 bytes from fe80::14c6:97c1:7a3e:b28f%en0, icmp_seq=1 hlim=64 time=51.307 ms
16 bytes from fe80::14c6:97c1:7a3e:b28f%en0, icmp_seq=2 hlim=64 time=66.994 ms
--- fe80::14c6:97c1:7a3e:b28f ping6 statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/std-dev = 51.307/112.132/218.095/75.200 ms