1.Is the word approximant just another word for semi-vowel?
2.Is this description correct for the sound /w/ in RP? /w/ voiced labio-velar approximant. Forget the 'voiced'. It can be at least partly devoiced after /t,k/, as in twin, quin, sqeeze.
3. Is this description of the sound /r/ correct in RP? /r/ voiced post-alveolar approximant Forget the 'voiced'; It's usually fully devoiced after voiceless plosive, as in pray, trend.
4. Is it correct to label the sound /r/ in RP rolled instead of approximant?
No. The approximant /r/ may be rolled/trilled in RP in oratory, (and it often is in some varieties if Scottish English), but it is not usually trilled in RP
5. Do we have just one post-alveolar sound(r) and just one labio-velar(w) sound in RP?
/ʃ/ and /ʒ/ are post-alveolar fricatives.