Don't remember to well, but I think I would split the second the loading bar appears. Never done a coop run before so don't know for that. As for Downfall, the only thing I've seen help is focus the enemies with panzers in the line, an MP40 spraying the line is the best thing. Then 4 PTRS bullets when the run out in the balcony. You can get a collat if you time them running out well.
I always timed it myself because it was never getting done. If you have sony vegas here's a guide that I follow for timing mine https://www.speedrun.com/cod5/guide/39gdh
This has ruined many runs for me especially on Relentless when Roebuck doesn't move up. I've never seen anything that would make it a consistent problem, but rather a luck of the draw.
I've tried to find guides for this but none of them show how to do it where you launch forward instead of sideways. The biggest example is at the start of Semper Fi after the assault starts and every run is of someone flicking their mouse to the right and then up. I've tried it on runs but I haven't figured it out yet. What keys do you hold/let go of to launch forward.