Re: If I "were" king instead of "was"
I would agree that it is probably easiest for learners to opt for the "were" form in standard conditional structures of the "if X were the case, Y would happen" kind, since it will never be "wrong" (though it does puzzle some native speakers who are unfamiliar with the grammar, and so may be considered wrong).
Since, however, such locutions are considered either incorrect or, at best, poor style, by many AmE
speakers, learners are probably best advised to use the distinctively subjunctive forms where appropriate.
On the other hand, the "was" subjunctive can be found in writers who are not generally considered poor stylists (recently I noticed instances in Berkeley and A.J. Ayer, for example).
Also, possibly, there is a distinction between the two forms (at least for some BrE speakers) which is not often discussed, but which can often be detected in e.g. radio interviews with public figures:
1. If it wasn't for the fact that X is the case, Y would happen.
2. If X were the case, Y might happen.
In the #1 structure, Y is presented as remote from actuality; whereas in #2, Y is still possible. My impression is that some BrE speakers (probably unconsciously) do tend more towards the "were" form when entertaining an open hypothesis, and "was" for a closed possibility.
(I use the example of interviews with public figures, because they involve frequent hypothesising.)
Not a professional ESL teacher.