No. If an item has a price tag then you're not expected to bargain. You are not expected to bargain in a grocery store or restaurant either. The main venues for bargaining are the local markets, where the price will probably be a little flexible, although not as much as in neighbouring countries.
No, not even the locals drink it. Rely on bottled water, which is cheap and available everywhere. If you want to reduce the plastic you use, many places will refill your bottle for even less than it costs to buy a new one, or even for free. In larger cities the tap water is OK to brush your teeth with, but be careful not to swallow.
It is wet between May and October and dry between November and April. The cool dry season - occurs from November to January. In the Mekong valley, temperature can drop to around 15 degrees Celsius and the mountain temperature drop to zero degree Celsius or lower at night. Humidity is low at this time of the year and the most visitors consider it the best time to travel to Laos.