I am aware of this question, but it and its answers either are not what I need to know, or are so broad on the subject I could not figure out the exact situation I am confused about.
What I need to understand is the use of a real future subjunctive mood in English. Take, for instance, the following sentences:
"If he doesn't stay true to himself, he is never going to get other people to join his cause."
"We three shall have a talk once Mary gets home."
It's clear to me that one should use the bare infinitive of a verb to form its present subjunctive, for example in subordinate clauses after verbs like beg and advocate, which is used for events placed in the future in spite of the tense's name.
Because of this, one could assume that in the sentences above the verbs/verbal phrases in bold should be changed to not stay and get respectively (since both refer to future, either immediate or distant/undefined, and could be considered subjunctive due to untruth of the sentences as of the moment of speech). Is this assumption correct?