Hi! I really admire you for taking a test like the GMAT in your second language! I could never do as well on a test like this in my non-native language.
The top score on Quant is 51, and you're already at 50, so there isn't much opportunity to lift your score on that side. As you've figured out, the lift really has to come from the Verbal side.
Two of your three scores in the Verbal section were in the high 20s, so I'm going to assume that that's your current level (and not the one score that dropped lower). In order to hit a score of 730+, you would need to get to about 39-40 (or higher) on the Verbal section (assuming you continue to score 50 on the Quant).
A score in the 40-range on Verbal is about the 90th percentile (ie, only 10% of all test-takers reach that level on Verbal). That is, a score of 40 is really competitive but it's also the case that you can still get a decent number of problems wrong. When you're having time management issues (as you said you are on RC), the first thing to do is to make sure that you're guessing immediately on ~4 problems in the section and saving all of that time to spend on other problems elsewhere in the section.
So on RC, for example, you could decide that you're going to guess immediately on one problem per passage. You can spread your time out on x–1 questions, so you won't be rushing too much and making mistakes on those. (And you can adjust as you see fit. If you have an easier passage one time and can get to all of those questions, go for it—and then maybe on another passage, you guess fast on 2 problems.) I've also had some students decide to do this on one entire passage (all 3 or 4 questions). That may be okay if you are going for a max of 39-40, but it's hard to score higher in the 40s if you guess on an entire passage.
I'd also examine another aspect of your how you're doing RC right now. I see this pattern in both native and non-native speakers, but I think it can have even more of a negative impact for non-native speakers.
For most of our school "careers," we were taught to read carefully and make sure that we understood every last detail of the reading assignment. On the GMAT, that training is used against us.
You actually
don't want to understand
all of the detail—because (a) you're not going to get asked about all of the detail and (b) the test is open-book. You can re-read the passage any time you want.
If I give you a list of 10 facts and tell you I'm going to ask you about the 3 of the facts—but you get to keep the list and can look at it while I'm asking you the questions—then sure, you'll read through it once, but it would take a lot longer to try to memorize or understand every single fact and that's not worth your time because you know you're only going to get asked about 3 of them. When I ask you about one, then you can go find it on the list and re-read it and take your time to think about it before answering me.
That's the same attitude that you want to take towards the GMAT: a two-pass approach. On your first read-through, you do want to understand the "big picture" ideas / the overall story that the passage is trying to tell. But when it comes to the facts and details, just note *where* the details are and *how* they fit into the overall store, but don't spend much time trying to understand the details of *what* they are yet. Defer that until you get a question about them.
eg:
paragraph 1: Species Y appears to be in danger of extinction
paragraph 2: details about why people think that Species Y is so threatened
paragraph 3: contrasts with 2; provides other details to show that Species Y actually might not be in danger of extinction after all
On my first read-through, I don't need to know or write down or even fully comprehend the details in paragraphs 2 and 3. I just need to know how they contribute to the overall story.
When I get to the problems, then I do my "second pass" to understand the specific details I need to answer this one question I've just been asked. For example, if I get a question about how Species Y appears to be threatened, then I'll go look at the details in paragraph 2. Or when I get a question about why things might not be so bad for Species Y after all, I'll know to look at paragraph 3.
If you're doing what I see a lot of students do—trying to understand *all* of the details on that first read-through, before you know what the questions are going to be—then you're spending time (and mental energy!) on things that you're never going to get asked about. That's not a great use of your time.
It's worth training yourself to take the "two-pass" approach that I described so that you're not wasting time on information that you'll never need.
In the SC example you gave, answer (A) is very close to correct. For the placement of "in England"—I would almost call this an idiom.
Let's say that we remove the opening modifier and just make the sentence this:
In England, life insurance remained until the end of the seventeenth century a specialized contract between individual underwriters and their clients.
This is perfectly correct—there's nothing wrong with it.
The problem arises when you add the opening modifier (Introduced by...century). When you have an opening modifier that is talking about some noun but never tells you what that noun is, then when you get to the main clause (the main part of the sentence), that noun should be the first noun mentioned. You don't interrupt the sentence with another noun first because it's momentarily confusing. England was introduced by...? Oh no, life insurance was.
Note: You can have what I'll call an "interior modifier"—like the old Beatrix Potter OG SC problem (it was removed from the OG a few years ago, but if you google it, you'll find it). The structure there is something like this:
In her recipes, which she created and test herself, Cristeta Comerford...
In this example, the "which she created..." modifier is modifying recipes, so it's still "inside" the opening modifier. And once we start the main clause/sentence, right after the second comma, the first noun mentioned is the person that the opening modifier is talking about.
That kind of nuance is really specific / nit-picky. And it does take a lot of time to learn a language to that level of knowledge. (That's why I said I could never take a test like this one in another language!) The resources that you're using are all good ones to use. The only other thing you might consider is some targeted tutoring around Sentence Correction in particular. You can learn these things on your own but it will take time and you only have a few months. Tutoring is expensive, so I don't typically like to recommend it, but it can help to accelerate the timeline when you don't have a lot of time.
And for any online resources that you're using—forums or whatever where people are discussing problems—just be really careful to find certain people who are really good at explaining things in a way that you can understand well enough to replicate on other problems. And then look for explanations / posts from those people and ignore everybody else. Sentence Correction, in particular, is really hard to explain clearly.