The first two questions are pretty straightforward but we agree that the 3rd one is a little vague. You first need to understand what the passage overall is talking about.

The 1st para states that modern historians are more focused on a self congratulatory path and have restricted themselves to questions of social hostory, etc.

The 2nd para states that these historians have repudiated the commitments of nationalism and scholarly detachment that could be found in their own works from the time of the finding of the AHA a century back

The 3rd para talks about historians who have a contrarian view to these modern historians and explains that these historians do read and appreciate the works of older hostorians (unlike the modern historians mentioned earlier in the passage)

The last para talks positively about the 2nd book mentioned in teh 3rd para

If you understand the passage this way then it’ll be very easy to answer the first two questions.

Q1) The opening lines of the 2nd para clearly state that modern historians have rejected the notion of nationalism, hence they would dnounce the same. C makes perfect sense

Q2) The answer to this question should actually come from para 3. The author praises the second group of historians mentioned in this para for giving due consideration to works of older writers so obviously the first group of historians (who are contemporary historians basically)were not doing this. Thus D cannot be true

Q3) This question is a little vague and can only be answered throgh elimination. Such questions will most likely not appear on the GMAT however.

Addressing your concern of accuracy we would recommend that you attempt the passages in the high difficulty section of the RC99 more from the point of getting general practice than from the accuracy point of view because most of these passages are way more difficult than the ones you will most likely see on the GMAT and some of the questions are also not very GMAT-like. This is one of the reasons we have discontinued the RC99 and replaced it with the RC practice sets.

So attempt these passages only if you have exhausted all other RC material with you and if you want to get exposure to passages that will be tougher than what the GMAT is likely to throw at you. Don’t get too worried about the accuracy rate.