None of these.
“Does a Water Elemental freeze when kept below 0°C?” is super inadvisable: Dungeons & Dragons is not a physics or chemistry simulator, and you're almost always going to run into trouble when trying to treat it like one. Also, #2 would definitely not be okay—see When a Question Changes Completely, Should it be a New Question? which suggests such a significant change to a new, different question should just be asked as a new question.
You have an actual scenario you're trying to resolve. It's got something to do with water elementals. It looks like you're either trying to overcome them and finding out your options, or you're trying to use them and you're dissatisfied with a ruling your DM made. In order to figure out this scenario, you're asking us very specific questions about water elementals' interactions with freezing.
Instead of that, you should just ask us directly about the actual problem you're dealing with. What's the actual concrete scenario that you bumped into? What's the actual concrete problem that came out of it that you're trying to address? Instead of asking us questions that might eventually help you solve that problem in that scenario, just tell us directly about that scenario, tell us your problem, and ask us to solve your problem. The entire thing. We're here to do that for you.
Our experience with XY problems is there's always more questions to ask because you don't have the complete solution yet, and they always orbit a context we'd rather solve instead. A physics-in-D&D question is highly liable to get closed as unclear, with people asking you to just directly describe what you're trying to solve as above.