Circuit Chase Alpha and both River missions on Alpha are infact not that difficult, I've done them all on official.
You need to invest time practicing these and if you have patience you WILL beat them.
Some tips for anyone struggling with these:
Alpha canoe tips:
1. Watch the youtube videos and study the route to take. DYOR.
2. IMPORTANT: Practice it on single player which has no lag. Dont try official until you're hitting the right times on single player.
3. Keep your finger on 'W' the entire time, unless you have some kind of correction to do.
4. Hug all the corners
5. The two S bends is where most people waste the most time, you need to get through these points quickly.
6. On official, its all about timing. On OC I had to wake up early, 4-5am and do the missions on official when the ping was good.
Alpha circuit chase tips:
1. Watch the youtube videos and study. DYOR.
2. Sprint off the start (hold right click, aim the beam as far as u can and release when timer hits 0)
3. The room where u fall into an air vent, make sure you drop off to the left, so when it shoots you up you can grab the wall.
4. The room with the 3 ledges after the vent: jump to the first ledge (w+shift+space), sprint to the 2nd ledge (right click) and sprint to the 3rd ledge (right click) - landing on ledges! - if you dont do this, forget it.....
5. The room where you run across the roof can take some time to get use to, but keep trying it and eventually you will work out a system, i personally flick my orientation around.
6. The big drop off you have near the end (after crawling on the roof), you want to use your right click sprint, its all about timing, hold it in and when you see the gap then release it to shoot yourself through the hole.
7. Sprint at the end when going through the speed rings.
8. Sprint anywhere else in the mission you can to shave off time.
9. Practice on single player, so you understand how its meant to work and how the grappling behaves under perfect conditions. Dont try official until you're hitting the right times on single player.
10. If you find you dont grab onto walls properly, this is lag! - dont even attempt alpha when this happens, because you do need a perfect run.
11. On official, its all about timing. On OC I had to wake up early, 4-5am and do the missions on official when the ping was good.