How far can you safely drill into 100mm thick brick for expansion plugs
I think 75% of the brick should be fine.. normally there is a "blow out" of the brick within 1cm of reaching the other side... but this is really dependent on the type of brick (cement, clay, etc), moisture contnet, age of brick, diameter of hole and strength of the hammer action of the drill

If it were me, I would drill to about half way, then add some water to the hole and start to reduce my force on the hammer drill in order to slow the drilling speed. -- do not attempt to drill if you have an SDS type drill since the hammer action is not dependent on your applied pressure. 

once the plugs are in place, you should have no issue hand tigtening the plug. just dont over-tighten