Why is Honda making a not very good coolant and insisting you use it exclusively in their own vehicles? Have you read the sticker underneath the hood? They're quite serious about it.
I'd buy AMSOIL coolant if it weren't expensive. I think they say theirs can be used for 150,000 miles or five years, which is not the kind of magical number they mention in their oil change intervals. Havoline XLC does the same. Both of them meet ASTM D3306, AMSOIL meets a couple different ones, but Havoline is a) available as a concentrate, because I don't want to pay a premium for American water and b) much cheaper.
No it's not. Use battery water, buy distilled or deionized water at a pharmacy or online.
Lots of OEM-spec coolants are available. Buy the coolant that your manufacturer says you should use. If it's only available as a premix, tough, but that's what it is, unless you want to flush the system and use a more readily-available coolant and the coolant is compatible with your vehicle's cooling system.
Water carries most of the heat in a coolant formulation, it acts as a solvent for the additives that do the corrosion protection, cavitation prevention, boiling/freezing point modification, pH balancing, pump lubrication, etc.