1. When a dependent clause starting with "because" is placed first in a sentence, use a comma between the two clauses. When an independent clause is placed first and the dependent clause second, do not separate the two clauses with a comma.
2. No. The comma before the last item in a sequence of items changes the meaning.
a. I will visit X, Y, and Z.
b. I will visit X, Y and Z.
a. means three persons and three different houses
b. means 3 persons and two houses: Y and Z live together