What are the factors to consider or to worry when we migrate to SQL Server 2017 from 2012
https://docs.microsoft.com/en-us/sql/database-engine/deprecated-database-engine-features-in-sql-server-2017 as a list of deprecated features for 2017.
2012 to 2017 yes. 2017 to 2012 no.
No, because you can't restore from 2017 to 2012.


The main thing to look at with your current database is what Compatibility Level is it set to in the 2012 instance? If it's below 110, and needs to stay below 110, then it won't function correctly, or at all.

The only way you'll be able to definitively determine what won't work is install a copy of SQL Server 2017 somewhere (with the appropriate licensing), restore your database to that new instance, and then start testing your application connecting to that new database.