Solved

how to improve pit-stops time

The pit stop time itself can't be improved. However cars can block each other in the pit lane (more details about that in Joeys advanced guide above in this subforum) and if both your cars enter pits together the second will have to wait until the first has finished his stop.

Good question. I know this was possible in the old version of iGP. So either it was dropped in order to make the game more ... streamlined for mobile users or the devs haven't found the time to work out the new version of this functionality, which is quite possible because first the app had to be finished and afterwards all the new users required a lot of server work to handle all that extra load, work which is still in progress I think.

Would actually be nice if those Offices would have some use for a change. Doubt it did secretly happen already though, my stops are the usual 4.somethings and I don't think this leaves much room for improvement.