I suggest you use SNAPTOGGLES instead. Less expensive, easier to use, and work great.
Don't know what a knox box is. These are really tough though, but still depend on the strength of drywall.
This anchor is very strong but you must consider (2) additional concerns: 1) what is the weight of the door? And, 2) supporting the weight of the door is as strong as your weakest link. If possible, build a sort of 2 x 6 frame around the doorway area. In other words: use a 2 x 6 or 8 x width of door header at top of opening and support it by one each 2 x 6 at each end and run them vertically down to the floor. Dress up with wood moulding strips of needed. This method would remove the need from ify ceiling support to a more reliable support from the floor or vertical wall around the doorway. Good luck!
Yes, they can be used to attach to metal as long as it is a hollow, metal wall.
This should work fine, it's rated for up to 300 lbs.
You don't know the condition of your drywall under the texture and paint. You could be attaching this 300 lb capacity anchor, to a 50 lb capacity drywall patch, done years earlier. Something that heavy, especially articulating, should always be attached solidly to the studs.
If the door is hollow they should work great. If the door is foam filled then that would be the wrong application for this type of fastener.
I'd use a 5/16" or 3/8" redhead.
I agree with you. They are outrageously expensive for what they are. I personnally use "Togglers". At about a dollar each they seem to me to be a stronger solution. If you have a plywood backing I don't see how they could ever pull out and there is no plastic involved in the weight holding capacity at all once they are set. I think they are the most elegant solution when hanging heavy items on a wall, and especially if there is a plywood backing behind the drywall.