Each is interpreted perfectly.
I don't know if you want a letter-by-letter proofreading, but I found some typos and such if it matters to you.
1.1.It is quite easy when you know how to do a thing.
1.2.It is quite easy when you have the knack for it.
knack = cleverness (intuitive or squired through practice) enabling one to do something skillfully
2.1. He rushed forward.
2.2. He darted forward.
3.1. I could not understand what was going on as I was standing on the extreme outer edge of the crowd.
3.2. I could not make sense of what was going on as I was standing on the fringe of the crowd.
4.1. What with the snowstorm and the telephone dead we are held up here.
4.2. What with the snowstorm and the telephone dead we are stuck here.
what with = taking into consideration, because of, as in What with all you have to carry, we should take a taxi.
5.1. The fortress was attacked on all sides.
5.2. The fortress was besieged.
6.1. Questions were showered on the speaker.
6.2. The speaker was besieged with questions.
7.1. The situation was getting worse and worse.
7.2. Things went from bad to worse.
8.1. We agreed on condition that he provided the necessary instruments.
8.2. We agreed on condition that he provided the necessary implements.
9.1. It never occurred to me that it would be my lot to deal with this problem.
9.2. It never occurred to me that it would fall to me to deal with this problem.
10.1. The roadside was lined with lime trees.
10.2. The roadside was fringed with lime trees.
11.1. You will have to be very exact in your calculations.
11.2. You will have to be very precise in your calculations.
12.1. The firm was flourishing under the new management.
12.2. The firm was thriving under the new management.
Thank you for your efforts.
- For Teachers