Deutsche Bahn (DB) is extremely efficient. They do not oversell intercity trains, the confusion would delay the train and impact other trains which is unacceptable, unless it is a weather/safety delay. The S-Bahn commuter trains are another story. while generally on time, you will get up close and personal with perfect strangers during high demand times.
Commuter and regional trains generally have no seat belts. I traveled on a high speed train from Frankfurt to Stuttgart last summer and do not recall seat belts there either.
The ticket you buy is good for a specific date. If you miss the train you had planned on you just take the next one (and maybe end up without a seat...). And believe me, DB will sell each and every ticket they possibly can...:)