If you believe an edit is too minimal; that is, the post still needs some work, selected "Edit and Approve". This allows you to make the additional edits you deem necessary to perform a complete edit, and finalise the edit. In contrast, simply rejecting the edit sends it back to a lesser state. While we want a complete edit, a partial edit is certainly better than no edit.
If you believe the edit should not be approved, but you still think the post needs some work, you can select "Reject and Improve" to perform your own edit.
If you think the edit should not be approved, it is, but you accept that it is probably just a matter of opinion; forget it and move on. If there is no serious issue with the post, there is no reason to enter further discussion.
If an edit is approved, and there is a serious problem brought on by the new edit, roll the edit back. Post a comment, flagging the original editor with @original_editor, to ask them why they made such an edit, raising your own issues regarding why such an edit was harmful. At this point, it is entirely possible that the original asker will disagree with your assessment of the edit being harmful. Should they roll it back, consider entering discussion with the user in chat. Do not engage in a rollback war; if you can not come to an agreement, bring it up in meta, or flag for a moderator, citing your original argument of why the edit is harmful.
That being said, I would like to address the edit in question. In your question, you state that
"To me, this edit doesn't improve the readability of the question and still doesn't fix all (minor) issues, for instance capitalizing "I"."
However, you rejected the edit on the basis that
This edit changes too much in the original post; the original meaning or intent of the post would be lost
These are two very different reasons. You can specifically reject an edit as being too minimal; in contrast, if you argue that it still doesn't fix all of the issues, I would interpret this to mean you think it has not been edited enough, which conflicts with "edit changes too much".
In this case, it appears that another user did agree. However, three users did not, and the edit was approved 3:2. To me, this edit looks appropriate. It has changed so much because it is correcting grammar. While some do not pay much attention to grammar, good grammar provides quality to a question. I agree that this edit was minimal, though I found a few inconsistencies apart from the lowercase i; some of which were grammatical (for example, "how I can" should be "how can I"), others were spelling and punctuation. I have proposed a revised edit to the question.