Davide Piotto goes by Airbag Killex.
Yes, Killex can be frozen.Specifically, the lawn herbicide in question can be applied effectively after experiences with freezing and thawing. The product's manufacturer nevertheless indicate that freezing may damage containers even when super-cold weathers do not harm contents. It is recommended to store the concentrate in a cool, dark, dry location inside a locked cabinet or closet.
Expiry dates on chemical products are normally stamped on the packaging at time of packing. Check the active ingrediant on the container, if it is 2,4-D then you need to dispose of it at a hazardous waste site. Products containing 2,4-D have been banned in a number of first world countries for the past 20 years.
That depends on what kind of weeds they are. If they are "broad leaf" weeds, you can use a herbicide like Killex that is for broad leaf weeds only. That means that it won't kill your grass. If it is a mixture of broad leafed weeds and grassy weeds, and you aren't worried about killing your lawn (as in you want to kill everything that's there), you can use a broad spectrum herbicide like Roundup. There are also soil sterilizers out there, but if you use one of those, be prepared that you won't be able to plant ANYTHING there and expect it to grow for at least a year, possibly more.