Well, some parts of this are easy. Mine has the same assembly at the top of the stack as yours, pretty close. That's just a filter inductor for the second string of capacitors, with some spark gaps to keep it from arcing over if the whole stack draws an arc, and some damping resistors so things don't ring so badly. That's also what all the little series R's in the stack are all about -- current limit in an arc case, which can otherwise be a bunch of amps (thousands) and enough to blow diodes. You are showing this wired as a positive output, like mine was, but I am providing another set of series caps for the AC inputs so I can turn the thing upside down because I want negative output for my fusor from this. I already have a gonzo 125kv positive stack swiped from an old Glassman after all (with my own driver, theirs was a disaster waiting to happen), and a bunch of smaller Spellman supplies that "just work".
My transformer isn't potted of course, and at some point I may take Cliff up on that offer of higher volt coils for the big Spellman transformer I have because it's a lot better made than the Wallis one appears to be (and smaller). Either is going to be much more power than I'm going to need here, and would put the "kill" in kilowatts. In my experience, it's harder to blow drivers with smaller transformers vs the driver ratings anyway, though the full wave stack connection does help prevent reverse spikes in an arc getting into the drivers and frying them. But nothing is ever perfect in that regard. Or I may use one of my smaller homebrew transformers, I've pretty much got that "going on" now and can make pretty decent ones here.
The nature of this kind of stack is that the bottom "ground" (your point C) will try to push negative as a load is drawn from the positive output, so a lead from there, with some series R to real ground will give you a current signal to monitor or use for control. It kinda-sorta looks like they are using that other diode and stuff to look at the voltage on the first stage, and extrapolate to the full stack from there, but I'll have to do more intense analysis on it to be sure. If you really believe in the bleeder resistor values in that second filter cap string, you could get a volt signal from the bottom of that by having some series R there to ground, and it might be more accurate (but with more time delay, making a closed loop harder to do). Of course in either case, you must carefully consider what will happen when there is an arc to ground off the top of the stack, the peak currents can be staggering, and a little spark gap or mov will simply go up in smoke, along with whatever it was protecting -- this is often the hardest part to get right in a full design. You WILL have arcs at some point in the process, so making things live through that is the most important thing.
I suspect that the stuff you show on the bottom has some error in the circuit tracing you did -- I've not seen 56 ohm, 100k and 1k resistors used in parallel anywhere else in these kinds of things. IT might pay to go look that over some more??? Some of that doesn't make sense at least to me.
I didn't measure any parallel R's in my stack, but then I only used a cheap DVM that wouldn't see 100 meg values at all. I guess I should now go and check that.
Cliff might chime in, but -- hey, he's the main honcho of a competing company, so let's not abuse him too much! He's been awful good to me and by extension, us, and we owe him thanks for what he's done already. Me, I just put my money where my mouth is and actually bought some stuff from them, and I'm glad I did -- it's rock solid and way worth it unless you time has very little value. One more subsystem with "no worries, it just friggin works every time". I got an SL2KW at 50kv/40 ma - way overkill for what I'm doing, but that can be nice too. Eliminates that nagging question of what would happen "if I just had a little more" once and for all, so I can get on with working on the real limitations. Half that power level would still be a good margin for what I actually do with the fusor. I'm only pursuing this project to get higher volts, not higher power, in my lashup. More than a few hundred watts just melts things and doesn't improve results at all.
BTW, you might want to put something up on the "introduce yourself" forum so we know who you are! This is intended for everyone to get a feel for everyone else, so we know at what level to ask or answer questions. Facilitates communication if we're not talking over or under one another! When we do that, no one is getting helped, which is the main point here.