I can completely understand your concern over not knowing what to study! However, I would strongly suggest you continue with InstantCert, even if most of it isn't meshing with the practice exam. I used IC primarily to study for this one and found it to be very, very helpful. The information was NOT irrelevant. You will need most of it.

I haven't seen the DSST practice exams you're talking about but I have taken other DSST practice tests and been somewhat unimpressed at the way the practice test compares to the real test, both in terms of difficulty and in content covered. From my own experience, I trust the material on InstantCert more than any of the DSST preparatory materials.

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History BA, TESC, starting Master's at AMU

CLEPS:
College Math-69, Sociology-70, Humanities-72, Am. Lit-61, English Lit-69, S. Sciences & History-65, Eng. Comp/w essay-75, Microec.-64, Macroec.-73, Western Civ. I-71, Western Civ. II-71, American Gov't-71, US History I-75, US History II-65, A&I Lit-77, Prin. of Mgmt-76, Prin. of Mktng-76. Biology-69

DSST's:
Civil War & Reconst.-70, Modern Middle East-63, World Religions-76, Tech. Writing-67, Prin. of Supervision-67, HR Mgmt-69, Rise & Fall USSR-64, Vietnam War-60, Ethics-69. Intro to Bus-67, Org. Behavior-69, W. Europe-60