Learn English – How long more must I wait

It was very kind of you to do the washing up, but you did not have to do it.

converts to

Your action was not necessary.

To have to is to be obliged to¹.

It was very kind of you to do the washing up, but you might not have done it.

converts to

It is possible that you didn't do it.

Might is "used in auxiliary function to express permission, liberty, probability, possibility in the past"²

It was very kind of you to do the washing up, but you must not have done it.

converts to

I have come to the conclusion that you didn't do it.

Must is "used to indicate logical probability or presumptive certainty"³

¹_{http://www.dictionary.com/browse/have-to}
²_{http://www.merriam-webster.com/dictionary/might}
³_{http://www.thefreedictionary.com/must}

As the others have said:

I might work more this week.

Implies that you have some sort of control over the situation, whereas

I might be working more this week.

Implies that you don't really have the choice.

can't i say : "There is traffic on the highway so i might arrive late."

Yes you can. And the reason this shows that you don't have control is because of the mention of the traffic.

Also context can play a big difference, if somebody invited me to a party in a week's time and I said

I'll be there, but I might be late.

That would imply that I have control, because I knew about it a week in advance, whereas if I sent that message on the day, it would imply that I have no control.

I hope this helped.

As for your second set of questions, both make sense. However I would have said "I have to work more today" to show that I have to do it, not that I really want to do it.