NO, you cannot modify programmed mission during its execution. You can pause it, do some action like take a photo, start recording and so and resume mission, or stop it.
Programmed Waypoint Mission is loaded into the memory and then executed. It is possible to pause it and make some photos or so and continue, or stop it. Any changes in waypoint mission need to be saved and loaded as new mission.
The possible way how to do it is to move WP1 to new required position, insert (Mission settings - WAYPOINT 1 Settings and green + icon on the left up corner) WP2 with WP1`s parameters at previous WP1 position.
Sorry for my English :)
To be honest I haven't got P3P, so I can't confirm what arrows are on the body of the drone (if they indicate rotation of the motors or propellers), but black dotted motors are CW (arrow to unlock propeller) and grey, or non dotted are CCW.
Never mind what mission you fly
Litchi - Camera Settings - Photo Settings - Capture Mode - Interval
If your Image Format is jpeg then Intervals start from 2sec, if Raw then 5sec.
When you start your Waypoint mission just tap on camera icon to take first picture and then others will be taken...