Kinda hard to say, depends on immigration where you do your things. Some take a 30 days extention in mind and let you have a return 30 days later, some don't. Better do it properly unless you know what they accept or not, you can usually move flight dates for a smallish fee or book a refundable one.
It is possible but you should get a longer visa to be safe. The new immigration boss is more strict than before, ED aren't that easy anymore but it will probably be ok if your passport in blank.
Cops may arrest and fine you 5-15k baht, you may contest as the law is about buying but dont do it as they can end your hollyday with a deportation stamp.
You can take it if you can afford the fine in case you are unlucky enough to be caught, use it in private area and you will be fine. Thai people dont care about it, a lot of them use it.
depends where you are, water work can be messy here and water can get contaminated. If everything works properly you can drink it in some area but even then it taste like bleach...