A mobile pass exists within the eurail app on the ONE phone it is applied to.

Reservations are not made within the app and are not directly tied to the pass.

You can make reservations on the eurail website by logging in with your details and linking your pass (this is not the same as activating it).

OR you can use many other (often cheaper and better) sources to make reservations. Read this thread:

Thank you so much 

Just to mention, you do not have to activate the pass in order to make reservations, it is best to leave it until you are sure you are travelling as it can be an issue to cancel/alter it once activated.

Also, please be aware there are issues reported with the eurail app not working properly for people from your part of the world due to the phone’s calendar being set to the Buddhist calendar.

You do not have to load your pass into a device to make a reservation. If you require a pass number the reference number in your confirmation will allow you to create a Pass Cover Number through the link below.

Eurail: https://www.eurail.com/en/book-reservations#/generatePassCoverNumber

Interrail: https://www.interrail.eu/en/book-reservations#/generatePassCoverNumber

As you are probably aware you can only add your pass to one device and any changes after activation need to be done by customer services

https://eurail.zendesk.com/hc/en-001/requests/new?

so should we set the phone’s calendar to Gregorian ? 

Can we do a seat reservation before purchase a global pass ? 

Yes, that seems to be required for the app to work correctly.

Not on eurail website as it requires you link a valid purchased (but not activated) pass.

On other sites listed above it is possible but do read their cancellation policies as you may not be able to get a refund of reservations if your plans change.