Mit dem ICE in die nächste Station einsteigen

Formally yes. The reservation ceases 15 minutes after departure, though. Although, as this is a train where you need a reservation anyways, nobody will suddenly show up and take your seats.

My friend just buy a sear reservation for like 6 or 7 EUR separately if you want to truly ensure you have a place to sit and are afraid that your seat reservation is taken. Or take the risk and then if someone is sitting in your place, approach them kindly and show them your reservation.

Don’t worry. It’s simple. What’s probably worse, is that there is a very decent chance that the train will skip whatever stop you need because of some sudden emergency or change in plan. Happened to me a lot that the train must go through e.g. Ingolstadt but then because of some emergency on the train tracks, they have to take another route. Though if this happened, you get your money back usually.