Will these fit BMW or Mercedes fasteners?

Yes, hex fasteners across most vehicles use these same sizes. The 2-10mm range handles the vast majority of common engine cover bolts, caliper hardware, and interior trim clips.

How durable is the T-type handle on stripped or tight bolts?

T-type handles give you more mechanical advantage than L-keys, so they apply force more effectively without rounding the fastener. The flat head design also improves contact in the bolt recess.

Can I store these loose or do I need to keep them in the box?

The box is just for organization—you can pull out individual keys as needed. Keep them together though; losing one from a set makes the rest less useful.