If a creature gains one of the listed abilities before Majestic Mryiarch's triggered ability resolves, perhaps due to another ability that triggered at the beginning of combat, Majestic Myriarch will gain that ability.

Majestic Myriarch's second ability triggers at the beginning of each combat, not just combat on your turn, whether or not any creatures you control have any of the listed abilities.

Majestic Myriarch's first ability applies in all zones. While it's on the battlefield, it counts itself, so it'll be at least 2/2.

Which abilities Majestic Myriarch gains is determined as the ability resolves. They won't change even if every other creature that has the abilities leaves the battlefield or if creatures enter the battlefield or gain abilities.

Multiple instances of any of the abilities Majestic Myriarch can gain are redundant.

If one of those creatures has one or more variants of the listed keywords (for example, hexproof from white), this creature gains those specific variants.