The best man is supposed to wear the same thing the groom does. The groom and best man often wear the same tie but it may be different from those worn by the ushers. Otherwise the two are dressed like the ushers.
Not necessarily. Usually the “head usher” is a person selected to take the bride’s mother up the aisle as the “head,” or “first” usher.
The ushers can divide their duties among themselves. The groom decides who goes on which aisle. One of them is asked to look for the bride’s coming and to notify the groom, another is especially detailed to take the two mothers up the aisle.