Will be married to a US citizen in June, my O-1 Visa expires in October, can I legally stay in the US after my visa expires?

Yes, you would be authorized to stay in the US once you file your application for adjustment of status.

You should avoid unnecessary travel after receiving warnings.

The DSO at your school should be able to resolve this issue for you.

Should we wait to have my wife's (USC) last name changed on all documents - Passport, SSN card, when applying for my Green Card?

Her last name is not the big issue in this case, it is whether you can prove that you entered the marriage to establish a life together.

Why USCIS wrote my prior EX wife name of on I797 NOTICE OF ACTION as applicant although i am the applicant

This looks like a mistake in the USCIS' system. Call customer service at USCIS.

I wandering if I can start my application for my legal status?

Your son may file a petition on your behalf on or after his 21st birthday.

What step should I take ? Is it possible to get an immigrant visa

You would need to contact a Canadian immigration attorney regarding travel to Canada.

My question is for how long in future she can switchback to H1B (different employer) provided she has approved I140.

Maybe one year and maybe three years. Consult with an immigration attorney.

Am i eligible for form N-470?

No. That form would have to have been filed and approved in advance of a planned departure.

Address this question to your real estate professional or CPA.