>> Just to reconfirm will this doesn't work for reservation, found this link in one thread here only:
I'm sorry, I can't help with Happy Rail. I tried going through their Glacier Express reservation process, but you have to provide all your personal details before getting to see what particular trains can actually be reserved, so I didn't persist with it.
I suppose it's possible that additional seats are quarantined for booking by third-party agents only, but I don't know for sure whether that is the case or not.
You could go through the reservation process on Happy Rail, and if you find that they can offer you a reservation on the particular date you want before you have to provide payment details, then you could proceed to payment.
If you have to pay before getting a confirmed reservation date/time, then I would abandon the booking process and send them an email to ask about availability on your travel date.
>> Should I reserve the regional train for the same route (as mentioned above I have Swiss Travel Pass)?
You can't reserve seats on the regional trains. With your Swiss Travel Pass, just board any regional train and pick any seat you like.
>> Is there any chance of not getting seat in the regional trains as well?
No, they run at hourly intervals throughout the day, so there are plenty of seats available.
There is only one Glacier Express service per day in each direction at that time of the year, so that is probably why it is already booked out.
>> I hope regional trains from Chur to Zermatt is included in the pass.
Where are you going after Zermatt, and how do you plan to get there? Are you planning to take any other trains with panoramic windows?