Yes, I believe so, per the last paragraph on page 24 of the rules. The three combats added the sorties to get it to 6, so there is no additional sortie for the final trace and no trace failure. However (obviously) the convoy can conduct no more traces until sorties can be removed in an upcoming replacements phase.
Correct. As long as the interdiction failed, the supply line trace succeeds regardless of its final sorties value.