Not sure why they don't seal the tops. Easier access I suppose. It is very nice to be able to just lift the top up to dump in some kalk. But it requires that the reactor be positioned in the sump or above the sump. That is good that they are now including check valves. I'm guessing other people probably had problems with the back siphoning.
The BEST way to dose the kalk is with a peristaltic dosing pump, which allows you to set the feed rate. Unfortunately, you will end up spending around $300 for one. You can keep an eye on the selling forums and ebay. I occasionally see a Litermeter 1 pop up there for a decent price.
In your case, I would do 2 things. First, add another float switch and hook up a relay. The second float switch will kill the feed pump if the water level rises to its level. This will keep you covered if your original float switch ever "sticks". Unfortunately, it still wouldn't have helped in my situation. Here is a link to where to buy the materials and a good tutorial. FWIW, I just bought a dual float relay setup from them for my new sump, but it is only for adding RO.
Another thing you can do is control the amount of flow coming from your feed pump with a valve, similar to how a calcium reactor is run. Set it to a slow drip that is just higher than your evaporation rate. So when the pump comes on, it is only a slow drip. If it sticks on for any reason, it will be nearly impossible to overdose the kalk.