1. Is there one huge mass of Void that reaches to Hallownest, Pharloom & possibly other kingdoms, but still ultimately just one finite body of "liquid"?

I like to think so, that the Void is a single entity that expands throughout the center of the world of Hollow Knight.

2. Is there anything solid below, and if so, how deep does it go? (e.g. in relation to the depth of GMS's cocoon)

If there is something below the Void, it would be great if it turned out to be the origin of the Pale beings. Like, immersed in an almost infinite sea of Void lies a pale, pristine core that is where all Pale beings are born, or at least where they get their divinity. That could explain the relative compatibility between the Void and the Pale beings.

4. Are the Void Tendrils creatures living inside the body of liquid? Or are they manifestations of the Void itself? If they are creatures, were they born like that, or did they turn into that form?

I seem to remember that, if not in Silksong or the original Hollow Knight, in the hunting journal the Tendrils were described as part of a single entity and a manifestation of its natural aggressiveness.

Not a theorist, but on the fifth question. I think 'my family' here just means White lady, Pale king, and Hornet. It would make sense if they were Godseekers or the shrooms (in fungal wastes), since it's explicitly told they shared a mind. We don't see anything like that in either game. Team Cherry is also very considerate with their choice of word. It would make more sense for White Lady to say 'my kind' instead if that were the case.

But either way. It's impossible to tell until we see another pale being that has no connection to the pale being we already know.

1. I believe it's a single thing mostly, maybe not an ocean necessarily, but something that has spread through tunnels in the ground and so. I do believe it's in every kingdom, the HK hunter's notes on the shade suggest that the void might be something that's some sort of left over from all bugs.

2. Probably yes, void might have not always been there, or perhaps not as plentiful, and I reject hollow planet ideas so.

3. No, and iirc team cherry said that HK took place all in a cave, so even Dirthmouth wouldn't be part of the surface (though perhaps close, if we are to assume Silksong's surface is actually a surface and the color scheme means something). Oh wait, I think you are talking about the surface of the void... I suppose it would mostly be since liquids do that, but that personally bothers me, I wouldn't be surprised if there are some separate "oceans" of void, but still not too far from one another that void creatures can still go from one to the other if they desire, unclear.

4. They are void given form, if we are to believe the game. I assume void has a "limited" ability to turn into something, but it clearly can be molded into something else, the tendril might just be its default shape when it stops being liquidy.

5. Hmmm I interpreted this as the king, the while lady and hornet, to an extent it might even extend to the siblings since the flower doesn't seem to repel void just by tough (the knight holds it fine) but by aggression. Pale beings seem rather selfish from one another (radiance and the pale king) so I doubt she talks about the pale beings in general, she would probably say "our kind" or something like that.

6. As I said I think void comes from regular bugs, Jiji mentions regrets, so perhaps it's something like their twisted, regretful soul when they die, idk, but I don't think its creation is tied to the creation of pale beings. That said, pale beings being the reason so many bugs just die from left to right may or may not increase the spread of void?