Could a 2nd EC private key be derived from a public key?
Given a EC public key, can a different, but plausible and functional private key be derived to match the public key?


No, a public key will correspond to only one private key (with one minor exception, which I will explain below).  With Elliptic Curve systems, the private key is an integer $d$ between 1 and $q$ (the order the generator point $G$), and the public key is most commonly the point $dG$, which defined to be $\underbrace{G + G + ... + G}_\text{d copies}$.

Now, the order of the generator has this property: all multiples less than the order will be distinct.  That is, if we consider the sequence of points $G, 2G, 3G, 4G, ..., (q-1)G, qG$, there will be no duplicates in that sequence.  What this means is that, since a public key occurs somewhere in the sequence, that is the only occurance; there won't be another private key (less than $q$) that will also yield that public key.

Now, the minor exception I mentioned: some elliptic curve systems don't use the full point $dG$ as the public key, sometimes they use only the x-coordinate as the public key.

It turns out, in that case, there is a second private key value that would yield the same public key; the value $q-d$.  There are no others, and that second key turns out to be of no use to an attacker; recovering that value is no easier than recovering the original value of $d$.