Please read this recent thread (search function!!!) first.
https://www.pianoforum.net/smf/index.php/topic,7984.0.htmlAs you will see, the perhaps most important advantage of a sight-reading skill is actually to be able to spot the most difficult passages of a piece right away. So you would do well in improving this skill!
On your first question: it is highly subjective. As you said, everyone has his problems with different techniques. Concerning your example, you have different techniques required. What is the most difficult? Depends on...you! Perhaps it is the hand crossing with the right hand jumping in the bass? Or the quite akward runs in the right hand (black keys! if I remember correctly...). Noone knows. You just have to test it, try it out.
Now we are back to the original problem, sight-reading. I mean, nobody expects you to play the variation fluently, even at slow speed. Just try to come through it, and make notes all over the score where you struggle. Also play hands separate, but when it comes to e.g. hand crossing, try at least to imagine how it will feel like. You could also just look at the score, identify the required techniques and then judge (from your experience) wether they are difficult to master or not (but looking at a sheet of paper and actually playing something on the piano are two different issues, that is clear). So it would depend on the techniques you already used and mastered (for example, you look at the score and see these arpeggio patterns. You think: hmm, two months ago I played this and that piece, they had exactly the same pattern...it shouldn't be so hard...). I think you get it.
Hope I was helpful a little at least.
Best luck,
Topic: Identifying Difficult Sections