How about:
"Just because I was mean to you, you should not be mean to me."

In spoken English it is fine ... maybe if you are writing it as part of a relatively formal report or something, then would be better to insert "for" .

Yes strictly speaking you would say "this coming Wednesday" for future events, and "this past Wednesday" for events in the past - but in most situations "future" and "past" tend to be omitted, as the timeline is implied by the context.
I went to the gym this Wednesday.
I will go to the gym this Wedneday.

I guess this is comes from usage, not a "rule". It is like asking, if the past tense of "cheat" is "cheated", why isn't the past tense of "eat" "eated".

I would say it has to do with the rhythm and implication of sound in a sentence. If someone says "I saw her for long", people would understand the meaning, but it would sound as if the speaker's thought was cut off.