Gosh, that IS a difficult situation! I'd start by being honest with yourself, with your weekdays effectively packed from 9am-10pm, and only weekends to truly devote to GMAT study, there's no way you're going to become an "expert" and comprehensively cover all the topics on the exam, so take a deep breath, and just let all that go.
You can't do it ALL in such limited time, BUT you CAN do SOMETHING (and a lot of something). Here's how I'd focus your study plan:
- Buy the OG 13th edition if you don't already have it
- Buy the GMAT Prep 3 and 4, and download the free GMAT Prep 1 and 2 if you haven't already
TODAY: Take the GMATPrep 1 and analyze, based on your weaknesses choose 3 of the MGMAT Quant guides to purchase. I'd suggest the Foundations book and the Number Properties book, then pick the third based on what you're getting wrong. Commit to completing all 3 books before your Test Day.
Jan 25 - Take the GMAT Prep 2
Feb 1 - Take the GMAT Prep 3
Feb 8 - Take the GMAT Prep 4
Feb 10 - TEST DAY
There is no way you can "only" study on the weekends, so try to work in a minimum of 1-2 hours of GMAT prep each weekday. This might mean you give up your lunch hour, or have to find a way to secretly work in 15-20 minutes here and there during your work hours, but you've got to do a little bit. Commit to having GMAT material around you at all times -- MGMAT books hidden in your work desk, GMAT YouTube videos on your SmartPhone, etc. Seriously, every 5 minutes you can grab is WELL worth it.
If you can't remember a certain topic AT ALL, Khan Academy videos (do a Google search) are great when you're on the go or are having a hard time learning a topic from the MGMAT books (they can be kind of dry). You may also want to check out Magoosh and Grockit to get more practice answering questions online.
Basically, follow the MGMAT books, starting with the Foundations, and do the corresponding problems listed at the end of each chapter in the OG.