Can I sponsor my parents as I am a temporary green card holder?

Your parents need to apply for visitor's visas, which in all likelihood be denied. You will only become eligible to sponsor parents for LPR status...

How do i know when its time to do medical after submitting my i485 application?

Once you receive notice of that interview appointment, you will know "it is time."

I agree. Anything smoked or inhaled within the prior 2 weeks will end up showing on your urine test.

If I were to travel outside United States in December, would that require me to appear for visa stamping interview again ??

Yes, you will need to apply for and appear at a new visa interview in your home country, assuming that H-1B petition will be approved.

Can I continue to work after filing for H4 COS

Can only work until the expiration date of your H-1B I-94. Got to stop afterwards.

Do need to fill out N-600 or N-400?

If either parent naturalized prior to you and/or brother having attained age 18, then both of you are now eligible to apply for Form N-600 Cert. of...

Visitor visa denial 214(B) no strong ties

It is "wise" to reapply only after having had an experienced attorney review her case and advise her to do so.

What is the best approach to change my visa status from H4 to F1?

Apply for the F-1 visa at the US consulate in your home country. Otherwise USCIS is sure to ruin your plans if attempt to apply for change of...

Can I apply for i601a ?

The I-601 A provisional waiver is only for those present in the USA. You need to apply for the regular I-601 waiver. Will take up to a year to be...

Is it gonna be a problem if I have 2 cases open at the immigration?

Should make no difference at this stage. You will have ample opportunity to withdraw that "U" application later.