1) 5hrs including 2 immigration stops (on both sides of the border) and 1 toilet stop. Do however make sure that your date does not coincide with a major public holiday when the highways are usually jammed and takes considerably longer.

2) Hotel Istana is located near the Bukit Bintang district in the CBD and if you take a taxi to LeMeridien it would probably take 15min under light traffic or 30min under moderate traffic. During peak/rush hours on a weekday it may take as long as an hour.

3) Since the coaches bypass the towns and villages, there isnt much of a view except for oil palm plantations and rolling hills. Best to keep yourselves occupied with a movie or music.

For drop-off proximity to the Hilton/LeMeridien at KL Sentral, FirstCoach http://www.firstcoach.com.my/ offers a very basic serviice and drops off near the Bangsar LRT station which is 1 stop on LRT from Sentral or 3-5min by taxi.
Nice Coaches (Plusliner) http://www.parkmayberhad.com/nice.html drops off at the old railway station which is also 3-5min from Sentral but hailing a cab may be difficult there.