-Is it true that all devices of the same type (MX, MS, etc.) bound to a network with a template upgrade their firmware at the same time?
"When scheduling an upgrade on a template network, all child networks will upgrade at that time within their respective timezones. For example, if a template network is scheduled for an upgrade at 7PM, all child networks will upgrade at 7PM in their configured timezones."
"Firmware for templates is managed on a per-template, per-product basis. Scheduling a firmware upgrade on a template will ensure that all networks bound to the template upgrade to the selected firmware version."
The way I am understanding that is if you have 10 MX tied to a template, that they will all update to the same firmware version per timezone. The per-product means that you could update MX only and you could do MS at a different time (unless I'm misunderstanding something).
-Is it true that firmware updates of all types of devices can only be delayed a certain amount of time but not suppressed totally?
This has been my experience, as I've had to push out updates multiple times 30 days out each time.
-When trying to set the date of the firmware update to a date two weeks from now the dashboard tells me that the "Firmware upgrade date must be set in the future". Bug or feature? Or did I overlook something? In this case it concerns switch software ("New firmware is available for this network. However, an update is not scheduled.") The date correlates with the Upgrade window that is set above.
This is either a bug, or your simply selecting a date on the wrong month or something. As long as the date your selecting is in the future it should work. It sounds like your seeing this information on the 'General' configuration page. I would go to Org>Firmware Upgrades and configure it there.
-The dashboard tells me "Upgrades may be staggered...". Is it true that this feature is in BETA? How do I access it?
This might vary per product. Usually switches will perform the upgrade with the 'furthest' switch. So the CORE would be last for example. Access points is a hit or miss in my experience. The upgrade itself has no impact, it is only when the device reboots that there is any downtime. That is unavoidable.
-For the whole firmware upgrade process: is there any integration in the API planned/already implemented?
None that I am aware of and I would wager they probably won't offer something like that for a while, if ever.