Professional standards are if the seal is broken then discard.
Storing the mixed product:
Fresh is always best. The basic thing with storing starsan type sanitizer is your judgment. I will co2 purge a keg that is filled with phos sanitizer (like star san) push that sanitizer through very well cleaned kegs and then keep that ~18lt sanitizer on tap in a sanitary vessel. The basic is if its crystal clear and the pH is below 3.5 than its good. Mine actually reads at pH 2.5 - 3.0. If its got bits in it then when its fresh its ok, but if can see tiny contaminants in it, or its discolored or isnt crystal clear anymore than its shot. I have also let a bucket sit after that stage and it gets pretty funky. If its used too much it will definately spoil. Cross contaminations include chemicals as well like dont mix chems at all! Dont think that brewing cleaners can mix together. If it gets any beer in it its spoiled as it will get cloudy and slimey and get slimey strings growing through it. Other more scientific people here and elsewhere could talk on that but basic is cross contamination will spoil anything including sanitizer itself.
Re- Basic Rule. When in doubt, chuck it out. Phosphoric Acid Sanitizer is awesome. When used correctly! but Its not bulletproof.
San regular is a mix to make it frothy and sudsy.
I use the 96% pure Phos, less bubbles and it goes further.
Starsan doesn't have to be crystal clear to be effective.
If it's within the pH range it's fine.