Can I obtain a TN VISA with a Green Card in Process?

Not automatically disqualify you, but will make it very difficult.

Why i got refusal for my parents visitor visa B1/B2

Probably because they did not present enough evidence that they intend to return to their home country after their visit. Either that or they said...

Make an infopass appointment and ask them this question. https://my.uscis.gov/appointment

How can my US citizan brother get my 23 younger brother (who has down syndrom) to USA as he has no one to take care of him

There is no easy solution to this, no special category for ppl with down syndrome. A sibling peittion takes 12+ years and then there is the...

Does the current B1/B2 visa become invalid if the application for renewal gets denied?

Not always, but depending on the reason for denial it can happen, yes.

This will be discretionary decision made by the CBP officer when you enter the US. Arguably, you can travel to the US if you don't have the intent...

Besides INA 212(a), are there any other grounds of inadmissibility for B2 visa holders are subject to at Port of Entry?

Do you have a specific question with real life facts? This is not law school, its a forum for ppl that have specific questions about themselves in...

How to maintain 2-year-greencard status if I have to stay away from the USA more than a year

Two issues, abandoning and lifting conditionality. You will need a reentry permit to be gone for more than a year, so file that before you leave. ...

Will me being on Food benefits automatically deny my fiance to come to US on Fiance visa?

You or another sponsor needs to be able to complete form I-864 affidavit of support - thats how you evidence you can support your spouse.

Can he sponsor me with this Specifications for the USCIS?

You have to look at the poverty guidelines for USCIS I-864 Form. Then see if the sponsor meets them keeping in mind that he supports spouse, 3...