Also, one of the reasons I want the timer is to try out all the different federal law enforcement agencies' qualification tests. But I have one question about the part of each test where they must do a magazine change. Do they have to change magazines on an empty chamber and then reload a round in the chamber, or do they already have one chambered and simply swap magazines? Like for example, the FBI test requires 4 rounds, magazine swap, then 4 more rounds. Do they have 5 rounds in that first mag and 3 rounds in the 2nd mag so that they don't have to drop the slide after they swap? To me that seems a bit cheap if that is in fact what they do. In the real world that wouldn't be the scenario, and could take away precious seconds if it has not been practiced.
You've already got several answers to your first question so I'll stick to the second. I can't speak for the federal LE qualifications but I can speak to local LEO/security and military qualifications which are all very similar.
What you are talking about are two different kinds of reloads. One is the speed reload and the other is the tactical reload.
The speed reload is used when you are in an all out gun fight and you run your gun dry (chamber empty, slide locked back.) In this case your only goal is to get back in the fight ASAP, so you just let the empty mag drop out of your gun.
The tactical reload is used when there is a lull in the action, or you have sufficient cover, and you still have rounds in the magazine/chamber. You are simply "replenishing" your gun with a full magazine. In this case you always retain the partial magazine for later use.
Every qualification I've had to shoot for different entities, have all included both types of reloads, but with the emphasis on speed reloads. Usually they'll have 3-4 speed reloads throughout and 1 or 2 tactical reloads at the end.
For example, I just requaled for my armed security cert. We shot at 3, 5, 7, 10, 15, and 25 yards. At the 10 and 15 yard lines we shot 3 rounds, speed reload, 3 rounds (two mags of 3 rounds.) Then at the 25 yd line we shot 3 rounds left barricade, speed reload, 3 rounds right barricade, and ended with a tactical reload behind the barrier. So for the 25 we had 1 mag of 3, and two full mags, even though we only shot 3 of the rounds in the second mag.
Hopefully that was clear enough and answered your question.