Some Problems in the Doors of Anime Destiny (with Spoilers)

The above part of PS is understood correctly, but these are all things that happen on the Alpha world line.

The animation begins with the beta world line, and Okabe enters the alpha world line after sending the first D-Mail in the middle of the first episode. The time machine of the alpha world line can only go back to the past, but the time machine of the beta world line is not limited by this.

Your first question:

What the beta world line wants to stop is the three world wars, and it needs to destroy Gurusu's paper and save her, which is different from the alpha world line-INB 5100 does not need to interpret SERN secrets. Therefore, the migrant soldiers never (nor need to) return to 1975.

Your second question:

(The initial state of Alpha World Line means that only Okabe's first D-Mail is valid, and the time machine can only go back to the past. )

From the perspective of migrant workers: from the future to the present, I went back to 1975 without knowing that the bucket was the father, got INB5 100 and lived on it, then gave INB5 100 to Akiba and died in 2000 (if I remember correctly). I jumped in twice and never looked back.

From the perspective of INB5 100, it was obtained by the soldiers who returned to 1975, handed over to Akiba, handed over to the shrine, and finally handed over to Okabe and Lisu.