Does the "come" in "come a certain time-point" need to conjugate according to verb tense?
"Come" in this construct is in the subjunctive. Fowler, in Modern English Usage (2nd ed.), s.v. subjunctives, gives the example "I shall be 70 come Tuesday".
This "come" is certainly not a preposition. It can't be coordinated with a preposition.
*Before and come the wedding-day, ...
*Come and after the wedding-day, ...
are syntactically impossible.