I just sent a challenge. The first two corners were pretty good, but I fucked the final one up something awful. lol
Writing down all that goes through my mind while doing a drift is hard lol.
I forgot that challenge. So yes, your two first drift were quite good and that's what I would do without some training on that track. It's possible to get more points on the first by overtsteering a bit more.
Second, you need to take even more the outside and less straight. You want your car to already be in the good angle to attack the corner. At the start I brake a little bit, then throttling and counter steer is needed in the middle of the corner;
As for whether it's possible to do this in one clean drift, the answer is yes, and sadly you won't see it on my ghost but I recorded it, with 3000 points. Gotta upload later ( I didn't validate the ghost because last drift ended in a spin and I don't want to teach bad drifting )
Last drift, you will see my ghost putting the front wheels in the dirt of the corner lol. It's not necessary. For this corner you need to brake at the begining of the drift, steer and counter steer when you have the good angle. Refer to my ghost for angles.