grammar - Past subjunctive Vs. Present Subjunctive

The verb be is the only one that has a past subjunctive form. In some cases, be, as a subjunctive, retains its present form even if the sentence is in the past tense.

Example:

Present: It is essential that she be present.

Past: It was essential that she be present.

However, in other cases, namely, conditional sentences and subordinating conjunctions, we use the past form of the subjunctive

Example:

If there were a death penalty for corporations, Enron may have earned
it.

So two questions here:

1. Is there a reason the subjunctive be remains in its present form in some past
   tense constructs, yet is converted to the past form were in some
   others?


2. And if we use the were-subjunctive to express contrary-to-the-fact
   clauses, then why Shakespeare said this?

If music be the food of love, play on.