a) because the previous managers left the defence in a horrible state which needed urgent rectification. The fact that he got that defence from being the 11th best in the league, to 3rd (only 3 behind Liverpool) alone should tell you how good a job those purchases have done thus far. However, it also tells you that more needs to be done to ensure that it's the best, as opposed to 3rd.
b) because Greenwood in the long-term is a striker, and not a RW. He doesn't play that role in the same manner as a proper RW does. We need a player who has the flexibility to go both ways there (down the byline and cutting inside to shoot), which we don't have on the left with Rashford, and which we don't currently have with Greenwood. Diallo looks like he might have the raw tools and talent for it, but we still need an experienced player there right now. We should be looking to replicate the depth that City have in those positions (Aguero, Jesus, Sterling, Mahrez, Torres, Foden, Bernardo) - we simply don't have that right now (or if we do, then it requires both Pellistri and Diallo making it which is very risky)
c) I think it's too early to say if it's a serious problem. It's been three games where the players have not had a preseason, but no excuses for the sort of performances we saw against Spurs. Managers have been sacked for them in the past. Hopefully Telles can fix that issue at LB and give Shaw a serious kick up the arse. VdB should now be coming in for Pogba, as he burnt through his credit during that Spurs game. Hopefully no Martial should also mean that we see a new system being tried out vs Newcastle. If we can make those changes, we might just see the tide turning.