It is probably not a good idea to reuse these containers for anything than what they were originally used for. If you read the label on any container used for chemicals, it states that you shouldn't reuse the container and to dispose of properly. Even if you rinse them out several times, you can still smell whatever was originally in them. If you put a new developer in the used container, it may get contaminated by whatever was in there before. It may not be enough to hurt anything, but I wouldn't want to risk ruining my negatives/prints. These containers aren't that expensive. My suggestion; buy a new container. I use my old containers to store used chemicals till I can properly dispose of them.

I have found that a measured one gallon does fall about 1.5 inches bellow the top of the bottle. My person thought is better more room than not enough.

Yes; it is HDPE (high density polyethylene) however I do not believe it to be BPE free, short, SHORT term storage is safe I'd think . However I doubt you could ever use for chemical storage following contamination of a food substance, plastic does leach (absorb) ...