Re: Mahr Question
If you are askinbg the religious opinion then according what i kow, you can demand whatever mehr you like, and marriage will not happen unless the husband agrees to pay it and it will be stated in Nikkah document.
If you want to know social aspects or your options, it is best suited in life1, so i will make a copy of it there.
This is a pure blackmail that your fiance is doing and you should not give in to it, remember that if wedding is called off, your fiance's family will suffer also and they will have tough time finding another match once the word gets out what they did.