Is there a minimum gap required between first & second visit using B1/B2 after having stayed 6 months in the first visit?

There is no a required period. There is common sense.

Can i apply for a J1 visa when OPT is set to expire in 10 weeks? I have a Masters from a US University.

The main issue here: is to find a sponsor willing to help.

How to protect myself, job and immigration situation for getting citizenship with applying for a gun and permit for carrying it?

The weapon permit will have no consequences for the citizenship process. For the job: it depends on your employer.

I 212 waiver

It will be a difficult situation. You will need to file a motion to reopen, if possible.

Adjustment of status.

If you are a US citizen, it does not matter if they had a gap in their status or not.

Should I choose option a or b on the I-485 form? Or h? Thank you in advance.

Paper file all. You need to file more forms however: I 765, i864, G325A, I 131 ...

Do you have a lease, paystubs, utility bills, credit cards transactions from CA starting January? Than you should be Ok filing now.

What are my chances in getting the waiver approved? Do i need to have a lawyer to help me apply for the waiver?

You need to work with an attorney. You do not have it, but it is much more complicated than filling out a paper form.

Re-entry for green card holders

Are you talking about a Re-entry permit? You should check instructions to form I-131.

Can this H1-B be used anyhow?

I do not think you can use that old H1b.