How to choose between "Subjunctive Mood" and "If - Adverbial clause of condition"?

As pertains to your first sentence, here (British Council: Verbs in time clauses and 'if' clauses) is an explanation of the principles to observe. It is a correct sentence, but the mood is not the subjunctive, it is the indicative (present). It doesn't matter whether the person you are talking about is absent or present.

The second is not correct from a certain point of view (mixed conditional) otherwise it is correct (conditionals) . Here is the other form.

• If he had come, he would have told me all the facts.

The difference between the two sentences—once corrected—is that the first relates to facts concerning the future, whereas for the second it is the past. You use "would" (ref.) for the past.

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The new sentence, after the correction

• If he came, he would tell me all the facts.

The subjunctive past (came) , according to some terminology, is used to state a hypothesis in the present and the near future (If he came now, or a little later…). The use of "would" is modal; it is meant to express a predictions or "refers to unreal or improbable present or future situations" (ref.).