Database Mirroring - Will forcing the service only lose non-committed data?
Correct, once the Primary loses connectivity with the Secondary (closes), all in-flight transactions will be rolled-back on the Primary and never make it to the secondary.  The committed transactions are guaranteed to be written to disk on the mirror server in that mode.

Here is an excellent article with more detail on this subject - 
https://www.sqlskills.com/blogs/paul/a-sql-server-dba-myth-a-day-130-in-flight-transactions-continue-after-a-failover/