Some clarification...
That second options (SINGLE ACTION FIRING - TRIGGER FORWARD) sounds like trouble to me! Also useless! Just my 2 cents worth.
It's not a matter of "useless", it's just a matter of how you operate the trigger (I'm only talking about the P99 here).
Here's how it works. If the striker is not cocked, but a round is chambered, then firing the pistol is done by a long, relatively heavy double-action pull on the trigger (Double-Action means that the pull on the trigger does two things: 1. cock the striker or hammer; and 2. fire the pistol). This is what has been called "TRIGGER IN FORWARD POSITION DOUBLE ACTION FIRING" in this thread.
It's at this point that what your finger does will determine which single-action mode you are able to do on the next shot (Single-Action means that the pull on the trigger does a single thing: 1. fire the already-cocked pistol). If you allow your finger to relax during or after the shot such that the trigger moves all the way forward, then you will have a long trigger pull with a cocked striker. This trigger pull is much lighter than the double-action pull when the striker is not cocked, but it is still a long trigger pull. This is what has been called "SINGLE ACTION FIRING - TRIGGER FORWARD" in this thread.
If, however, you train your finger to keep the trigger fully compressed to the rear during the recoil (good follow-through), and then let the trigger go forward just until you feel a click, then you can fire again with a short, light pull on the trigger. This is what has been called "SINGLE ACTION FIRING - TRIGGER REARWARD" in this thread. This aids in doing accurate shooting, and to me is the best way to fire shots subsequent to the first one. Now, it does take practice and training to get your finger to do this each time. It is easy to let the trigger relax all the way forward. Give it a try and you will see what I mean.
BTW, the Glock action is somewhat similar in that the first pull is a long double-action shot, but the next shot depends on what your finger does, just like I've described above. If you let go of the trigger, then the next shot will be double-action again (there is no cocking or decocking on a Glock). However, if you only let the trigger move forward until you feel a click, then it is a short, easy trigger pull.
Sorry that this is so long-winded.
I hope that it clarified some things.