This is not an immigration question, I'll re-classify it.

You forgot to tell us the expiration date of the combo card.

Can a H1B visa filed by my previous employer under consular processing be transferred to future employer (H1B-not activated)?

I'm confused. Did you leave the US when your work authorization expired? Normally you need to work for the first H-1B employer for a few pay...

What are the obligations of the co sponsor for i130 petition for alien relative ? i mean do they have to put a bank guarantee?

Look at form I-864P for the numbers and form I-864 for information for your joint sponsor.

How long does it take to issue AP and EAD combo?

I'm assuming that you also filed form I-131 and I-765. Yes, the COVID-19 situation seems to be causing a delay beyond the 'normal' 90-120 days...

If your H with the 'older' employer is still current ... contact the new employer and tell them to withdraw their H transfer papers.

What's the best job option for me to get a H1-B Visa?

I recommend that you NOT go to a community college ... go for a 'real' school for your 4 year education.

What will be a time duration (no.of days) to convert from H1-b to F1 or F1 (CPT) within US itself ?

A COS to F-1 takes a very long time ... as my colleague pointed out. PLUS you can not get a F-1/CPT work permit until after the F is approved...

My co-sponsor only has tax return for 2019, can't get the transcript right now, is it ok to present that only at the interview

Is it a real photo-copy of the tax return that she filed, with her signature? Or, it is a re-creation from TurboTax, or some similar...

You seem to be confused. What is your current immigration status?