"I want to be a swimmer."
Since I had to look this up myself: the "ni" particle is used in the sense of moving toward a future destination. With a verb like "become", you move toward what you are becoming: a swimmer.
英語で「せんしゅ」のいみはどのいみですか? ちょっとだけ、上手もにがてもそうみたいんですか? In english, what does 'senshu' mean? Is it like, jouzu or nigate, where you have a certain amount of skill, good or bad?.
is swimming supposed to be pronounced suiei or suiē ? If the former, then why?
I don't mean to get complicated, but that depends on how you pronounce "suiei" and "suiē". You could be pronouncing them completetely different or exactly the same, so if I pick one merely based on how you chose to write them, it might still end up being the wrong actual pronunciation. All I can tell you is that it's generally written as the former in the Latin alphabet.
Obviously, すいえい would be the best indication of it's pronunciation, but I can image that even for native speakers the precise sound of those graphemes might differ depending on their regional accent.
My bad, I was wondering whether it's pronounced /suiei/ or /suie:/ if that makes more sense. An い after え usually results in an elongated /e:/ vowel, such as in 映画, but here the audio makes it sound like the final syllable rhymes with "may". I know it is correct not to elongate the /e/ in ～ている verb endings but I'm not so sure about nouns.
Sorry for the late response, and yes, that does make sense. I believe the elongated /e/ in Japanese really has an /i/ sound at the end instead of /e:/ (with the exception of things literally written with a double ええ of course). Small caveat; I have not studied the 'true' phonetics of Japanese, and dialects might still muddle this distinction.