So, it's 3% ammonium. I think that since you're using exceptionally small amounts, it's not something that you can't handle. Like I said, your nitrifiers are dealing with ammonia all the time.
They likely mean that it's 39g of K per 100g solid. 52% of the mass of KCl is K, so 39g of K is equivalent to 75g of KCl. Seems a little low, if you can find something that is more pure, it may be worth it, just to get more bang for your buck, so to speak.
As for the expiry date, don't worry about it. KCl isn't going anywhere. It melts at 775oC and doesn't boil 'till about 1500oC, so you needn't worry about it evaporating. It will absorb water over time, so yes, technically the proportions of things will decrease as water increases, but the same compounds will still be present. It's just that since the amount of water has increased, there will be less KCl per gram than before.
That's interesting though, I just checked what I've got, "NoSalt", Ingredients: KCl, potassium bitartrate, adipic acid, fumaric acid, silicon dioxide (sand), and mineral oil. No ammonium.
Also, it's 96% KCl (650mg K per 1.3g serving).
I were I you, I would be tempted to find something more pure. But I don't think that you need be concerned about health, it's just easier to dose if you can assume purity.
