Update:
After researching for removing stripped phillips head screws, I saw this youtube site:
https://www.youtube.com/watch?v=rpbL8h-7jSY&list=RDCMUCT7uKjjnhi3eozGVrjKfTzQ.
Interesting ways of removing stripped screws, especially using rubber band! Anyway, I was abled to removed two screws that was attached on this CARB diagnostic control mercedes 015 545 43 21 (California-only part related to emissions diagnosis) as showned in the first picture below. The previous attempt to removed the two screws as mentioned in my last post that I thought stripped both screws. I used another phillips screwdriver and it was a tight fit, thus removing both phillips screws with ease after putting in pb blaster on both screws. After that, there was definitely more room to play to get that last nut inside the brake booster! (see second and third pictures below and noticed much more room to play on the top left corner thread where the nut goes in) After putting in the pin that goes inside the brake pedal with the brake booster fork, I then finished up the remaining connectors on the brake booster, including the asr pre charged brake line, as well as the remaining brake lines on the master brake cylinder. See pictures 4 and 5 below.
I then poured brake fluid dot 4 inside the brake fluid reservoir above the master brake cylinder and filling it to the top. After starting the engine, I kept pressing the brake pedal to feel if any traction on the brakes and that did not happen. Instead the brake pedal went straight to the floor. So I kept the engine running for about 10 minutes and pressing the brake pedal off and on. Shortly thereafter, I felt traction on the brake pedal and the brake pedal stop just about 2 inches from the floor. I guessed it took some time build up pressure on the brake system. I put the car in reverse and sure enough the brake pedal stopped the car from moving after pressing the brake pedal. I guessed it took some time for the brake fluid to activate on all four wheels. It seems, so far, the brake booster, master cylinder and all the brake lines works! I checked the brake fluid level on the reservoir and it shows a decrease in the brake fluid level, not much but enough for me to put in more brake fluid dot 4 to fill it up. I also checked the brake lines around the master cyclinder for leaks and so far no leaks.
The next thing for me to do is bleeding all 4 brakes as described in the technical article:
https://www.pelicanparts.com/techarticles/Mercedes-W124/67-BRAKES-Bleeding_Brakes/67-BRAKES-Bleeding_Brakes.htm
I will continue to follow up on my progress. Stay tune and thanks to all for reading and posting comments on this thread.