You do not need reservations for any of the trains you intend to take. Just sit anywhere you want, provided the seat has not already been taken by someone.
If you reserve a seat, and then miss the train (for whatever reason) that you reserved a seat on then you can still travel on another train, but you will not have a reserved seats. Reservations cannot be modified.
Most Lucern to Interlaken trains are “PE”. They have been labeled that way because they have large windows, and this way the railways can trick tourists in to believing they need a reservation (which they are happy to sell to you, as that makes them a lot of money).
The IR trains run the same route, but use older stock. There are only a few of those, and they serve as relief trains for the regular service (that is why you see them running s few minutes before the PE trains).