Question about Garmin Forerunner 55

It does not have an altimeter.
But it has a great battery life and ingenious conditions,

Hello,
Yes, but you need to install an option on the Connect IQ Boutique. I've added altitude and cumulative D+. You can only add 2 fields.

Not directly, but if you use the corresponding function to track an activity, GPS is also switched on and then you also get altitude tracking.