Question: Wouldn't Thanos have been able to kill all of the Avengers and Guardians on Titan without trouble as he had 4 stones?
Answer: Well Dr. Strange used the time stone to see how they could fight him and they executed that plan, though as Strange knew they wouldn't win against him. Eventually Thanos did beat them but I suppose advanced technology and strength in numbers were a good advantage even if he had 4 stones. I think they intentionally went for speed and his mind as that's the 2 stones he was still missing.
Answer: Since they are in possession of the Time Stone and Doctor Strange is hiding it with magic, he may have decided to refrain from killing all of them, otherwise locating the stone afterwards could prove a difficult task.
Answer: He could have, but Thanos sees himself as merciful and doing the right thing. He fights the Avengers and Guardians in order to stop them and gain the Infinity Stones. He has no particular interest in killing any of them.
Question: Were they originally going to have Scott and Clint in this movie?
Answer: I've read that the opening scene of "Endgame" with Clint and his family was originally going to be in this film. Ant-Man's inclusion seems doubtful since the post-credits scene for "Ant-Man and the Wasp" takes place at the same time as the climax for this film and Ant-Man is on the other side of the country doing his own thing.
Question: Instead of telling Stark to turn the donut around why doesn't he use a sling ring?
Answer: The range of the sling rings hasn't been defined. They were in deep space at this point, so they may have been too far away for Strange to open up a portal on Earth for them to step through.
In Endgame, Strange opens at least one portal from across the galaxy. For example, we see the one opened from Titan that he, Spiderman, and the Gaurdians all come through. Unless his sling powers suddenly increased after the great purge, he should have been able to open a portal from deep space on the ship.
Answer: The Q Ship was travelling in some form of hyperspace. It would be reasonable to assume that the slingring could not work under those circumstances. When he does use it later, it is from the surface of the planet Titan.
And when they arrive on Titan his idea to protect the time stone changes. He instead looks at different time-lines.
Why would it be different in hyperspace? The ship has its own gravity.
By definition, hyperspace exists outside normal time and space. Depending on how the sling ring works, it may not be able access normal space.
Answer: We aren't given a specific answer, but as Thanos himself tells Tony Stark, "You're not the only one cursed with knowledge." Thanos has probably studied the Infinity Stones for most of his life, and with the exception of the Soul Stone, every Stone presumably gives off some kind of energy signature that can be tracked.
Phaneron ★