“A) I have one 1099-MISC and one W-2 as an individual. Can I get an extension for the 1099-MISC (w form 4868) and file my W-2 using turbo tax by the deadline?”----> NOT the Form 4868 which is for 1040. 1099-MISC is different than 1040 and needs a DIFFERENT form. You can get an automatic 30-day extension of time to file by completing Form 8809. Under certain hardship conditions you may apply for an additional 30 days.
“B) If I file for an extension for my 1099 (w form 4868), does the total tax liability (line 4) include or exclude estimated amounts that I will be writing off?”---->As said above; you need to file Form 8809 for your 1099-MISC.
Please visit the IRS Website here; http://www.irs.gov/pub/irs-pdf/f8809.pdf