That's correct that IR is not adaptive. IR questions are generally difficult -- even more so because you have to go all parts of that question in order to get credit. However, the scoring is such that you can get a few questions wrong and still get a "perfect" score of 8/8. So when you are presented with lots of difficult IR problems in a row -- focus more on time management. If you are almost absolutely sure that you you will be guessing on all 2 or 3 parts of a question -- then just cut your loss in terms of timing and move onto the next.
Of course, during practice, you want to build your confidence to a point that that does not happen. But if that's what you are presented with during the exam -- then the important thing is to move on and don't let it affect your psychology for the remaining portions of the test.
For more of our insights into the IR test, please see:
https://www.gmatpill.com/gmat-practice-t ... -reasoning