SOLUTION:

For DBMirroring to be able to use KERBEROS,
you need to register an SPN for the service account to the Endpoint port number ! 

I finally got this to work lowering AUTHENTICATION settings

..;   FOR DATA_MIRRORING (
         ROLE = ALL,
         AUTHENTICATION = WINDOWS NEGOTIATE ,
         ...

It depends: https://www.brentozar.com/sql/sql-server-san-best-practices/

it depends on the nature of the data.
If that data is to sensitive ( names / Social numbers / ... ) they could refuse the auth.
IMHO in all...

I liked the article until I read the content of the DTU calculation webpage. 
At least the first 2 paragraphs are yours.

You can copy the data/tables from to your primary mdf file.

How? That depends on your current configuration.
If you post your databases DDL (create database statement) we may...

it actually performs the collation translation, and materializes that data in tempdb,  so .... yes.

The only decent way around it, is to create a new db and its...

Check the "remarks" section and  "Errors Unaffected by a TRY…CATCH Construct" in the try-catch documentation : https://docs.microsoft.com/en-us/sql/t-sql/language-elements/try-catch-transact-sql

No. local software is updated and scripts are provided to upgrade the db at startup (failover) time.
At startup (failover) time upgrades are being checked and executed if needed.
If...

of couse the best way is to avoid it !
GUIDs have their place in the db world, but there are a number of better options when just picking them...

( Starting with SQL2012, ) you could opt to move tempdb to a local drive on each node. ref: http://www.sqlservercentral.com/articles/failover/137066/

Each  node  has to be hotfixed ( cumulative...

As a picture says more than 1000 words: Statler & Waldorf ( and I hope everybody still knows the duo )

As a starter I always use the target versions Data Migration Assistant (formerly known as Upgrade Advisor ).

Have a look at https://docs.microsoft.com/en-us/sql/dma/dma-overview