Hi Sergiu,
1) Regarding leaving questions unanswered - Agree that it is a complex algo. But I have read in some places that
each unanswered question takes your percentile down by 3. Not sure of the authenticity of this, but if it's even slightly true, it is a very big hit. It could be the reason you scored low, but may not be the only reason.
2) My experience with GMAT Prep - I am taking the GMAT in a couple of months and I took the GMATPrep test (the first) after 2-3 weeks of study (almost full time). I was extremely surprised to see 760 even though I got 14 of the quant questions wrong and 4 of the verbal. Your score depends quite a lot on
which questions, what difficulty, how many in a row, how much time you spend at the end (whether the software thinks you are just guessing if you answered 5 questions in 5 seconds, say) and so on and so forth. Just a couple of days after that, I took the gmatclub test and scored a 670 with an abysmal verbal score - so bummer.
3) Data sufficiency - that's a new monster to understand. As you said, you'll start to
recognize patterns -
don't assume integers if not specified, positive does not include zero, percentages and actual values behave very differently, it's OK to have two equations in three variables if you are not asked the values of the variables themselves but rather are asked something like "what's x+y+z?" and so on.One of the keys is to
completely ignore A when you evaluate B the first time. It is pretty logical, but I agree some of the problems are tough to get your head around. Especially as you move on, you'll find the OG problems get tougher toward the end. Practice will help here.
4) General - I experimented solving a lot of problems, but my sense so far is, it helps to
pick a topic (like how Manhattan gives you),
master it, solve all problems you can find on that topic and then
move on. Also, what I call
"meta-study" is important - you have to
go back, revise what you found difficult/what you got wrong and remember why you committed the mistake you did. Finding patterns and remembering them is very important.
According to me, solving 20 problems and really, really spending time understanding them inside and out is more beneficial than solving 100 but not revising solutions/reiterating strategy.
5) Future study plan - Since I am pretty much in the same place as you are, here's what I am going to do, see if it helps you too: I have read Manhattan CR and am halfway through SC. I am going to finish a quick read/solving of these. I am going to solve OG the first time entirely on my own with what little extra reading that happens side by side. I am going to go through
gmatclub Math Book (available for free) once. THEN, I am going to repeat Manhattan CR, SC and gmatclub Math book. This time, I will try and remember strategies, specific equations etc etc. and underline important stuff while I revise the books. Then, I will go through all problem explanations in OG (the entire OG explanation section for all categories of questions) to see which ones I was doubtful about, which ones I got wrong, why, and even the ones I got right (to see if a better approach was available). Once this is done, I will take up OG Verbal Review and Quant Review with the aim to really, really reduce the error rate compared to the first pass of OG done earlier based on the study and revisions done so far. I will probably intermingle this phase with Manhattan's 6 tests that I got with the SC guide / the 2 extra tests I purchased for GMAT Prep /
gmatclub tests (if I am able to accumulate kudos, so there's how you can help

)
I think going topic-by-topic will be helpful - read the theory, immediately try out problems. This should help really "fix" it in your mind. Manhattan has the list of questions on each topic from the OG that I will try and look at the next time I read the guide (I only have the SC and CR guides,but still).
One final point -
I believe really spending time (even as much as 4-5 mins) on a tough question and getting it right is really good, compared to moving on every 2 mins. You only have to worry about time in the last 10 mins (PROVIDED your usual experience is that you are able to do questions in an average time of 2 mins - oh yeah, make sure you time yourself every time you solve practice questions). Some questions hardly take a few seconds, so you can use the extra time on tough ones and getting the tough ones right is a good boost.
Hope this helps! Best to you for your GMAT!