We don't pronounce the very last consonants of a word as they should be in English. "nước" is pronounced as /'nʊək/ without the /kə.../ sound at the end of it. Try saying /nʊək/ then stop immediately when you reach the end of the word. Don't make the extra /kə.../ sound which is like you are gargling or clearing your throat.
No, they should be treated the same way as mentioned above. And don't also try to link /-k/ in "nướC" with /é-/ in "Ép" to make a sound like /-ké/. Rarely do we link words. For example: You would properly pronounce "Can I have a bit of egg?" (sorry, this is gibberish to Vietnamese people) as "ca-ni-ha-va-bi-to-vegg?", we just simply don't. : )
You pronounce the two words together exactly the same when you pronounce them separately. The raised tone in "ớ" and "é" are at the same pitch and they should always be that way. And yes, you should keep the tone raised. Don't start from the flat tone and raise it since that would be like you pronounce two words instead of one. For example: "cá" is pronounced as "cá", not "ca-á" :)
Yep. Every liquid that you can get from the juicing process can be called "nước ép" (juice). For examples:
nước mía (ép): sugar cane juice
nước táo (ép): apple juice
nước trái cây (ép): fruit juice
You can omit the word "ép" (juiced/pressed) when the liquid is clearly a fruit juice. However, if the fruit is processed in ways other than juicing [say blending (xay), boiling (luộc, nấu), fermenting (lên men), etc.], you should keep the word "ép" to distinguish between different products of different processes.
I don't know the reason either. What you said never happens to me.
Next time you see this question, you should hit the REPORT button and tell developers that you're marked wrong for a correct answer.
Please also check if the app wants you to repeat or to translate what you hear. Sometimes, you don't get the marks for misunderstanding the tasks given.