Will DB's be able to get the +12 helms?

Nope, it's a hardcoded exclusion for all non primary-adventurer-classes. Untill Nexon America Changes this, and opens CwKPQ for everyone...

There are several ways they could code the PQ to limit it to adventures. The easiest and fastest way is to divide the job ID by 100, round down, and check from there. If it's 4, they're a thief. DBs would fit that.

Code:

if ( int(get.JobID/100) == 4 ) { pqPosition = THIEF }

Doing anything more than that would be time consuming. There's no reason to, so unless Nexon makes a specific effort to keep Bladers out of the PQ for an unknown reason, they should be able to do it just fine.