Too much bleach in res. would kill the plants?
Greetings, too much chlorine, above 1 ppm is not really good. Not sure if your residual from sanitizing is got you above that number. The PH is def an issue.
Mass Die-Off/Sickness: When you transplanted says to me that the new media/system was toxic in some way. Could be as simple as transplant shock and low RH or as complex as X, Y and Z compounded by C and antagonised by B...
PH Swing: In hydro is common and yeah daily swings are normal until the system is established. When I ran DWC I could control swings with the amount of bubbles introduced into the system. Low bubbles and PH dropped, crank it up and PH went up. You could also need to add some ca:mg to the system to try and get it to buffer better.
Nute Line: You are using sux as a buffer. One of the rare times that I say an overpriced bottle line is good at something, but (an) bottle line is good at something; I know works well in DWC for it's buffering.
Cl Based Sanitizing: I would use H2O2 instead. IF you do use Cl, I would not use more than 1 capful per gallon, and would never use it on hydroton, inner buckets i.e. closed system because of the surface area to be cleaned and residuals left in the system. The H2O2 will break down after a day or so to plain H2O.
These are all just general remarks,
not to be taken as a 'definitive' anything.