Hey, do we need to fully record a race for it to be valid or is the end card sufficient? Could be nice to have a small guide if you have time

Yes, a fully recorded race is required. Under the game rules it says "All runs must be submitted in video format All runs must show from the time when the screen says GO until final time is displayed" so the run doesnt need the 3..2..1.. countdown just the GO screen and an unedited segment from the start to the end

I just had a FLAP run were I got world record on the second lap then exited out of the race during the third. Does this make it invalid?

Nope, it's still valid! just as long as it shows the clear beginning of the race to end of the 2nd lap