1) On my set up, I have regulator, solenoid, check valve, bubble counter in that order. My main issue is that the water in the bubble counter pushes up to the reactor over time and when off, it runs back into the bubble counter, but I have never had it siphon back or be a major issue.
2) I would probably go to 6 drops a second, 4 sounds like too little, I have drops fast enough that it looks like a pulsed stream and I add 3 bubbles of CO2 a second, but I use the 3170 which is smaller and was never made available for sale and it has no pH port, I instead dose CO2 anytime the tank pH is above 8.10. It does keep up with my 215 when paired with kalkwasser, though I do tend to once a month have to make very small corrections with additives, KH may drift down to 8.5 and try for 9-10 and Calcium may drift down to 390 and I try to keep 420-440. This is mostly because the 3170 is in tank and algae grows in the tubes from light exposure for water intake and drip, so they clog and have to be cleaned so it is neglect on my part and one of the technical reasons this unit was never sold, I am thinking of trying black tubing.
3)It looks like a leak where the thread of this fitting enters the reactor, I would put teflon tape on the fitting and rethread it into the lid. All this fitting is is a gland nut used to water proof cable entries, they tend to be reliable as a seal on the probe, but the thread into the reactor could be allowing a leak.