that the net weight either cannot be the amount of actual coins staking

I talked with @sc-steemit and they said that based on the information given the numbers I used from the block explorer are the number of coins actively staking. If the documentation on this in the source is poor and it turns out that the number used does not accurately reflect the total number of coins staking, then I agree my numbers are void. Looking forward to the reply from the Blackcoin devs.

or you cannot use it to derive your chance to stake

The only way that could be true is if, on the whole, coins to not have equal stake weights. I have no reason to believe that is the case.