Well, whatever bubble rate you had before the water change should be the same rate more or less what you set it to after the water change. The KH of the tank water does not matter. For example, a bubble rate of 3 bps puts exactly the same amount of CO2 in the water whether the tank water was 6KH or 9KH. This will also not affect the colour of the DC. A 6 KH tank water with 30ppm CO2 will cause the dropchecker to be exactly the same colour as a 9KH tank water. The CO2 simply off-gasses into the air space of the DC and then diffuses into the 4 KH water.
Naturally, when you do a water change, and if you clean/reset the DC, then yes the colours will be all wrong, but that doesn't matter because the needle valve setting will be the same. Water changes actually introduce CO2 into the tank. The tap usually has plenty, and when the leaves are exposed to air they gather CO2, so it doesn't matter that the DC will take a few hours to come up to speed. Energize the solenoid and you'll be back on track.
I'm probably stating the obvious but when you do the water change you should simply de-power the solenoid, not close the needle valve. That way it preserves the needle valve setting.
Cylinder valves and needle valves are not perfect. As the cylinder empties the needle valve output drifts forcing you to make slight adjustments to the needle valve. This is definitely irritating and that may be the true value of brand name regulator components as opposed to the knock off ebay brands. Since I'm too cheap to pay for name brand regulators I can only go by what others have reported, but I suspect this is true. Some people report a higher incidence of BBA for example, which may in the long run be traced to an under-performing needle valve.
I agree with what you say, but one of the keys to better technique is understanding what the tank is telling you and to know how to identify the imminent warning signs. Imagine, for example those who are not aware that BBA, filamentous and often GSA are CO2 related. Or imagine those than are not aware that structural failures and deformation are CO2 related? Unless there is a sudden and obvious CO2 malfunction, the tank will tell you what's happening before things get out of hand. When you understand the signs, take immediate action but don't panic. Reducing lighting combined with CO2 injection rate increase will typically save the day.
You just have to decide that CO2 monitoring and adjustment is integral to your daily procedure and to treat it that way. If I spend 30 minutes every morning on the tank, 10 minutes is dedicated to removing leaves/debris, 5 minutes to fluffing/preening, 2 minutes to dosing and then the rest is checking/adjusting CO2. In my tanks, the first warning sign is typically a strand or two of hair on the highest leaves waving at me like a pirates skull & crossbones flag. I'll immediately make a slight upwards adjustment of the needle valve. After a while, you do get a feel for how much adjustment of this valve you need. In the end, it really is the plants themselves that talk to me, not the DC, which is entirely consistent with my "do-not-depend-on-test-kit" mentality.
CO2 is so critical in a high energy tank that there is a wide gamut of failures attributable to it's misapplication. There is a category of symptoms for slight, subtle failures, another category for medium or chronic failures, and another category for acute (bonehead) failures. This is what prompted Barr to suggest that if you have a plant failure in your tank, then there is a 95% probability that it's one of those categories of CO2 misapplication.
Hope this helps.
Cheers,
Good food for thought for those growing lawncovers. When I finally setup the Dutch, I'll heavily consider placing the drop checker near the HC to closely monitor whether the levels are adaquate down there. 
