Regarding your questions:
1) This sentence is not wrong, grammatically speaking, but English native speakers don’t speak like that, so it doesn’t sound very “natural” in my opinion. I would say something like this: “I was wondering if I could get your help with English because mine is a little rusty. Thank you!”
2) My native language is Portuguese so I 100% understand why you wrote that sentence the way that you did. The right way of saying it is “My biggest problem is, actually, comprehension”. That “the” is not needed in that case.
3) In this one I would use a different approach but please keep in mind that mostly native speakers would understand you, a more proper way of saying it would be: “Sometimes I have difficulties understanding, like when reading a passage from a book. I also have issues when it comes to Oral comprehension”
4) It is correct!
PS: let me know if you need any extra clarification with anything or if you need any extra help :D