Should there be a comma in front of "und" in the following sentence?

With the comma it looks like the following:

NO COMMA

SIMPLE LISTINGS (einfache Aufzählungen)

"Facebook, Twitter und Instagram."

DISPOSED SUB-CLAUSES (unselbstständige Teilsätze)

"Ich bin schon lange auf Facebook aktiv und nutze Instagram erst seit Kurzem."

SEQUELED SUBSIDIARIES (gleichgeordnete Nebensätze)

"Ich bin gerne auf Facebook, weil es wichtig für mein Marketing ist und weil ich Spaß dran habe."

YOU CAN CHOOSE

If two separate sentences are connected by an "und", you can freely decide for or against the comma. This is true even if the subject is the same:

• "Ich erstelle das Grobkonzept**[,] und** mein Kollege übernimmt später die Ausarbeitung."

The only important thing is that the second sub-sentence after the comma can stand alone as a sentence.

COMMA MUST BE

INSERTED ACCESSORIES (eingeschobene Nebensätze)

"Ich hoffe, dass ich Ihre Frage damit beantworten konnte**, und** wünsche Ihnen ein schönes Wochenende."

INSERTS (Einschübe)

"Gerne helfen Tanja, meine Mitarbeiterin**, und** ich Ihnen weiter."

ADDITIONS (Zusatz)

"Ich muss darauf unbedingt reagieren**, und** zwar schnell."

In your example, I would leave a comma and not a dot before the "und". Then with "Einerseits" and "Andererseits" you refers to something that is better dealt in one sentence. But because it is a subordinate clause, one could also put a point:

"Einerseits, weil es halt einfach so ist. Andererseits, weil es auf Wikipedia steht."

Yes, the second comma is needed, because the part

weil es halt einfach so ist

is a subordinate clause. You need to start and end a subordinate clause with commas if they appear in the middle of a sentence.

In this case, the actual main sentence is incomplete, this is probably an epiphrase to a complete sentence that stands before this one.

Nevertheless,

Einerseits [...] und andererseits [...]

is the part of the main sentence that is still there.

Yes, the comma is obligatory here – because of the preceding subclause.

Not because of und, however. It's about the subclause weil es halt einfach so ist. The comma rules for subclauses apply: if a sentence begins or ends with a subclause, a comma is set in between to separate them from each other. If the subclause is interposed, however, it begins and ends with a comma.

A comma because of und can only be placed if und connects two main clauses (but doesn't need to). However, in those cases other punctuation devices are often better suited (e.g. a semicolon, a dash or even a full stop).