Epic but left me several questions

Rating: 8/10 (28 April 2019)

I love the movie, i just hate that some questions are floping into my head after watching this movie:
1) Since they can bring back 2014's Thanos, Gamora and (twin) Nebula, why don't they just travel the time and bring back alive Nat and Tony? Hulk said the past isn't your future right?
2) Gamora is the victim of the soul stone exchange but proof can be bring back, why not black widow??
3) 2014's Thanos has been killed, so the timeline wil change after the civil war? Why the needs to bring back all the stone by captain America then?
4) How can Thanos shrink his giant starship with only one tiny prim material??? Isn't tranporting one man must wearing a custom shrink suit??
Confused.....