Are these two results contradictory?
No. It is what we've been talking about. T-Mobile apparently has set (at least portions of) their network up to not allow IPv4 fallback on devices that support IPv6.
This means you cannot resolve IPv4 addresses natively. The DNS can translate it for you, so you can access servers which are only served via IPv4 (70-80% of the Internet) but you must use a DNS name so that the T-Mobile DNS server can do its proxy trickery.
I'd bet if you try to use that link to Google I provided with only their IP address, it'd fail.
But, that's not what you need to test. You need to test from your home computer that serves MC. If it does not show
native IPv6 support (no Teredo or other transitional routing cleverness) then you wouldn't be able to use JRemote to access your home computer via IPv6 even if it
was fixed.
That was the point I was making above. Just fixing JRemote is only a tiny portion of the issue. You would need a fully IPv6 chain accessible from your home machine through to the Internet, and then through to T-Mobile, for it to work. This is possible, but isn't likely unless you've done a lot of work on your home network to make sure it would work (and have a home ISP which is prepared in your service area).
However, from what I've read, setting up a dynamic DNS account for your home machine and accessing your MC server that way through JRemote (instead of using an Access Key) would almost certainly work to solve your issue. It would have the side effect that you'd need to connect to a different server in JRemote when on your local LAN (unless you can set up Full NAT in your home router, and you probably can't) but that doesn't seem like the end of the world.
So, you'd have two servers in JRemote: (1) set up with an Access Key for use on Wifi, and (2) set up with your dynamic DNS hostname for use on cellular.