You need to accurately log the travel you make on your pass, not doing so is technically seen as not travelling with a valid ticket and there is a (very slight in this case) chance of having an issue in not doing so.

All you need do on the pass is de-select the single train and add in the correct one. You do not need to re-enter the whole day.

Reservations are not compulsory on that ICE beyond Mannheim so all you need do is be aware of any seats that may be reserved for someone else beyond there.

Assuming that this is the same train and you are just staying on for the extra stop then you don’t even need to deselect the existing Paris-Mannheim leg.  Just add in a leg from Mannheim to Frankfurt.

Thank you so much, you guys, that is really helpful

And since you are so helpful @zagmund and @Al_G , could I ask one more question as an absolute Interrail newbie: Is it true that I should only activate my journeys when I am standing in front of the train or can I activate them in advance, knowing that I can de-activate them one day before?

Thanks so much :)

P.S.: Worked wonderful to just add the leg Mannheim Frankfurt to my journey :)

It’s mainly important to wait to activate a travel day (your first journey of the day) until just before boarding your first train of the day. Other journeys on that specific day don’t really matter, since you can easily log different trains without any issues. Something can always go wrong in railway land, or one can get suddenly ill, so you don’t want to waste a travel day :)

Thank you so much, will do as you say :)