>> Based on SBB's timetable, there appears to be 2 options of route for the train ride from St Moritz to Zurich. One via Landquart and the other via Chur.
Both routes go through Landquart, but only one of the routes requires a change of train there, which is the train that does not go through Chur.
>> The one via Landquart takes slightly shorter travel journey. I believe both requires us to change train at the respective transit at Landquart or Chur.
Yes, that's right.
>> For us to choose 1 between the 2 options, appreciate advice on the following -
(1) presumably there is only one railway station in St Moritz and both train departs from the same railway station in St Moritz
>> (2) which is easier for us to manage in terms of changing train bearing in mind we have luggages to manage?
Changing in Chur is usually done on the same platform, from one side to the other.
Changing in Landquart involves going down a ramp to the underpass and walking along to another platform and up another ramp.
Neither option is difficult.
>> (3) what is the scenary for the respective route viz-a-viz, which has scenary different from the Zermatt to St Moritz via Glacier Express?
If you arrive in St Moritz on the Glacier Express, you will have already travelled along the route from Chur to St Moritz. If you wanted to go back to Zürich a different way, the train that involves changing at Landquart is a good option.
You can find some information about this trip, and some photos, here:
(My two posts of Feb 28)
>> Separately, is the Swiss Half Fare Pass the same as Half Fare Travel Card, which according to SBB's website allows holder to 50% discount to the above train ride.
The "Swiss Half Fare Card" is for tourists and the "Half-Fare Travelcard" is for Swiss residents. There is nothing called the Swiss Half Fare Pass, but people might call it that informally.
If you buy a Swiss Half Fare Card, you will get 50% discount on this ride.