Urriak Uruk
Gaming is fun, and fun is for everyone
EDIT: After reading the OP, I think the biggest issue that should be addressed is the difference in planeswalking and typical D&D magic. Planeswalkers travel through the Blind Eternities (or Aether) were horrible monsters reside (eldrazi). The process can drive you mad even (IIRC). So to me, that sounds most like the Far Realm. So I think planeswalking uses the Far Realm to bypass space time. However, this is very dangerous and only those with sparks, natives of the Far Realm or possibly gods can do it. However, the further you have to "travel" in the Far Realm to get to another world / plane the more dangerous it is or possible impossible.
It's a good point. I'd argue that many D&D worlds, like Greyhawk and Toril, have safe routes through the Phlogiston that make planar travel far easier, through safe routes like Spelljammer. But a world like Theros or Ravnica does not. The Phlogiston is a treacherous place to travel without the safe routes, and contains Dark Regions that no one returns from. So my explanation largely assumes that if you aren't Planeswalking, traveling through the Phlogiston will kill you or drive you insane.
So in this view, the Phlogiston = Blind Eternities. I especially like this, as the Blind Eternities is described as being filled with mana and Aether, which could be various forms of the highly flammable energy the Phlogiston is filled with.
Note, I'm not actually debating you in a "I'm right your wrong!" way, I'm just trying to justify my alternative idea. There really is no official justification for how these connect.