Leechers would have trouble destroying Shardblades
and they would die before they could do anything to Nightblood (as they/it is much more invested than even Shardblades).
Generally the speed at which leecher leeches is dependent on burn rate, and they are simply much slower at this than Nightblood. Also chromium burns relatively fast
On the nicrosil, that is a good question. I think Nicroburst would not really do anything to Nightblood, as Nicroburst affect only kinetic investiture, whereas Nightblood's effect seems to be inherent to it, i.e. it is a passive one.
