1)Yes, higher NRR is better. It stands for Noise Reduction Rating and it's actually quite complicated. The number is an average over several tested frequencies. I'm not sure what might be considered "safe" but I know none of them are high enough for my tastes. Consider that (basically) prolonged exposure to anything over 80 dB is considered potentially harmful, while a gunshot can easily be 130 dB. So, even if you have a NRR of 30, you're still looking at 100 dB. Also, that 30 is an average, so some frequencies are less, some more. I've never researched which frequencies are most prevalent in a gun shot or which, if any, of the products block those frequencies better. That's why I recommend plugs and muffs for practice, maximum protection. You should also note that the affect of wearing double protection is not strictly a sum of the ratings. In other words, 30 NRR plugs and 30 NRR muffs do not add to a full 60 NRR. Also, I don't know what the limit is, but it's probably not a lot higher than 35-40. At some point, the noise is literally reaching your ears THROUGH YOUR HEAD rather than through your ears, so plugging your ears better doesn't help any more.
You'd need a NRR helmet at that point.
2)Sorry, I can't help you there. Last time I looked at the various products I had the same issue as you. The manufacturer and the sellers sites are frustratingly vague and the information is tough to find.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.