I'm real life you can't tell whether it crossed the plane. But go on, spell out the answer to what becomes four scenarios . You can also address the ball that straddles the plane of touch if you like
Blue kicks high for touch
Red player positions himself to catch the ball, with one foot in touch ,one foot in the field of play... And..
1. Ball crosses line of touch, Red fumbles the catch knocking it forward, ball lands on field of play
line out as if kicked straight out.
2. Ball crosses line of touch, Red fumbles the catch and ball goes backward landing in touch.
Touch as if kicked out on the full. The catcher/fumbler is in touch.
3. Ball does not cross the line of touch, red fumbles the catch knocking it forward, ball lands on field of play
Knock on by Red
4. Ball does not cross the line of touch, red fumbles the catch and ball goes backward landing in touch.
Line out Blue throw.
If the ball straddles the plane of touch I would treat it the same as a ball touching the line. On / over the line = in touch. So scenarios 5 & 6 are the same as 1 & 2