First off, switch to either all distiller or all spring water for your non-wort water, it doesn't matter which one you use. Personally, I use spring water. With not having the two different waters issue, you should have no reason not to just boil them all together.
Second, you don't need to boil your rehydration water. If it's bottled, it's perfectly clean and most likely will not infect your batch.
Third, the plan you described is not a good one. If you let the boiled water sit around, you've essentially negated any benefit you achieved from boiling it, unless you can seal it in an air-tight environment. Regardless, you want to rehydrate your yeast in warm water, so you would have to reheat it anyways.
Fourth, rehydration is not a necessity, you could just put it right into the fermenter dry. If you have the ability to measure 1 gram of yeast for rehydration then you have the same means to measure out the same amount and just drop it into the fermenter dry. Rehydration is a good step if you have any concerns about the viability if your yeast, because it will give you a visual indication of whether the yeast is healthy or not, but it is not necessary.
Finally, having read your prior thread, I'm still not convinced that you have an issue with infections. Your sanitation practices are fine, you should be more concerned about getting high quality ingredients and letting your beer sit long enough in both the fermenter and the bottle before passing judgment on whether it came out well.
