1. Yes. I use warm(ish) water, but I also use an electric stirrer so I'm not around to see it mixing.

2: Yes, can't see why not.

3: No idea, I'm afraid. It seems to last long enough that I discard it because it's used up, long before it breaks down in any way.

4: 10% metaborate would be 20g sodium metaborate dissolved in 200 ml (=20cl) of water. I use solution because I don't have dry metaborate, so I have to make it from sodium hydroxide and sodium borate. I really prefer the bicarbonate version.