Hi, Do you have a special form to fill out a Withdrawal petition for fiancée, or it is enough only written letter from me?

You can write a letter indicating that you wish to withdraw the petition to USCIS.

Join college in TPS Status

This would largely depend on the admissions requirements of any particular school.

Do you think joining the army makes my green card be late? And also on my BCT, invalid I-20 will make me in trouble?

Please rephrase and resubmit this question for the best answer.

Will this complicate their TD visa re-application at Toronto US Consulate. Will the consult ask for the 221g documents?

Yes, any previous visa refusals will be the subject of inquiry at the Consulate. Be prepared.

N-400 biometrics

No, this would be a very unwise course of action. You can always reschedule your biometric appointment if necessary.

Is this $6300 deduction available to those who are working under an F1 visa using their OPT?

Consult with a tax attorney for the best answer.

How should one determine the tax residency/non-residency status?

This question should be addressed to an International Tax specialist for the best answer.

You cannot file the H-4 EAD until all the prerequisites have been met.

U-Visa Holder becoming a Permanent Resident through marriage?

I am not sure why attorneys would discourage you from getting married. Schedule a consultation with an immigration law specialist.

Will i be allowed to enter US with my current employers visa and new employers petition

It is not clear what type of visa or petition you are referring to. Consult with an attorney to avoid making a big mistake.