1) The important part of the ammonium chloride is the ammonium. So no, it can't be substituted.
2) The fixer can be used full strength or diluted 1+1. I prefer full strength.
3) pH of the finished fixer is below 9. If it rises above, it gets "smelly" with a strong ammonia reech. I aim for about 8, but measure only by smell.
4) The reason for the alternative alkalis is that I would prefer bicarbonate, but also prefer to avoid carbonates and bicarbonates when an acid stop is used. So metaborate is a good, safe alternative.
5) I don't use this fix with POP or lith prints. Like all rapid fixes it has a very slight bleaching effect in the highlights, especially with warm tones.