1. No.
2. Top 5 or 10之類值得考慮.
3. yes iff you are already very sure about your interest, and have a prof (in the targeted field) in another university had accepted you already.
4. Fight for the best research job in the freedom world for academic excellence.Else go to get a job in the industry for decent living (applied research).
建議有打死不會回國內卷的心理準備, 否則有你好挨的.