That's all fine, it's just not synced up. I'm just not sure where pressing 5 and 3 come into play, like you mentioned. Pressing 5 in the replay just seems to immediately pause the demo and 3 doesn't seem to do anything.
It is supposed to work like this. You create your first cam point at, lets say, 30 seconds in to the game, the second one at 35 seconds, 38 seconds etc.
After you have completed your camera path, save it and reload the demo.
Before the game time reaches 30 seconds (when you created your first cam point), press the key "5".
Now the "autostart" is active and the demo is supposed to pause all by itself at the 30 second mark.
Hit "numpad 3" to set the camera as your viewtarget, because that does not happen automatically, for now.
When you now unpause the demo with "F", the camera is supposed to start and adjust its speed to fly through the second cam point at 35 seconds, third cam point at 38 seconds etc.
This way, if you pointed the camera on a player whenever you created your cam points, the camera will later as well, point towards the player while it flies along the camera path.
This is as far theory goes.
Let me fix that bug that breaks the first few cam points, and I'll upload you all the files you need for this to work. Because you'll need certain keybinds and a demo manager that allows you to move during pause, otherwise you won't be able to create a proper camera path.