I still have a valid Visa. Can I come back on OPT? I have a desi consultancy ready to write a letter for me.
-Yes, you can come back. If you have a job, you may travel and reenter to resume work at the same job or you have a written offer for another job.
(Source: http://www.ice.gov/sevis/travel/faq_f2.htm#_Toc81222043)

Also how many days can I be unemployed in OPT?
- 90 Days unemployment for regular 120 days for STEM
- If you travel outside while unemployed it will count against the 90/120 days
- if you travel  while employed during a period of leave authorized by the employer, the time spent outside the US will not count as period of unemployment 
(Source: Page 13 and 14 , http://www.ice.gov/doclib/sevis/pdf/opt_policy_guidance_pdf.pdf)

If  I take a new I-20 for January, how many days ahead I can come to US?
-You cannot come more than 30 days before the start date of you program
(Source: http://www.ice.gov/sevis/factsheet/100104ent_stdnt_fs.htm)

Some suggestions:
1) Get a letter from your employer that clearly states your date of employment, job title and their intention to continue employing you after your return
2) Make sure the start date and name of the company on that letter matches the information your provided to your school (ie. the info matches what is in SEVIS) and you haven't had more than 90 days of unemployment
3) Make sure  your job title is an obvious one i.e there is no doubt that your job is directly related to your field of study