they could very well offer 4 to 6 different keystone runes just in that one path. or any number really. the system is more expandable than the old mastery system.
To an extent, that's true. It's possible that a couple slots have more than 3 options, but it's unlikely that it becomes the norm. Not for technical reasons, you're right, there could be any number of Runes in a given slot. But for maintenance and balancing reasons, it's most probable that they will limit themselves to the minimum viable number of Runes that still gives interesting options to everyone.
if the resolve tree is supposed to be aimed at fighters, divers and juggernauts,
They VERY pointedly and explicitly explained that no Path was targeted at a specific Class, and that no Class was expected to take always the same Path as Primary. I talked with Sparkle, saying that despite their best intentions, that's still how people would perceive the system, and well... here we are, you're the living proof of what I claimed. But still, Juggernaut and Divers could totally take Domination Primary, that wouldn't be absurd, even if that's "the Assassin Path".
heck, they could probably make a couple generalist runes that you could pick into any path.
Once again, technically true, but seems very implausible to me...
the "weapon coatings" keystone they showcased there as a concept could fit into any of the pathes depending on the stats youre enchanting your items with.
Everyone else already said it, but you don't seem to believe them, so I'll add my voice to the choir, for what it's worth: really, you shouldn't pay attention to the Runes in the video, it's anything but reliable information. The video could've been shot a few weeks ago with a Rune that has been sniped since then, it could be placeholder art with no real Rune ever implemented behind, we don't know.