Based on the information you provided, you will likely be denied entry on a visitor visa.

Do I need Indian visa?

You should inquire with your nearest Indian consulate about visa requirements.

How do I get started in the paperwork to bring my fiance to the USA legally?

You should consult with an attorney for the options available. A fiancé visa may be the best although the process is long. Otherwise after you...

Can we cancel a pending AOS case?

It is likely best to hire an attorney to do a review of the case and determine if the level of mistakes would warrant canceling the petition.

No a prenuptial agreement should have no impact on a K1 fiancé visa filing.

Is boyfriend in the same city that a TN visa holder is planning working considered intent to stay, preconceived intent, or ties?

No this should not indicate preconceived intent on your part unless you inform the officer otherwise.

No a boyfriend in the same locale should not suggest that there is preconceived intent on your part, unless you explicitly inform the officer of such.

Can I extend my B2 visa ?

It is most advisable for you to consult with an attorney about your possible extension, especially given that you plan to marry and adjust status...

Can my son law apply for me to get my permanent resident status?

If you married your husband after your stepson turned 18, then he cannot apply on your behalf.

How long does it take?

No you must wait until you receive your EAD to work legally in the US.