I am going to focus more on the idea of cell phone seizure because the other two questions were already answered to a reasonable degree.
We live in a day and age where cell phones are becoming more and more advanced and people are entrusting more of their personal information to these devices. In a very real way an officer of the law can learn much more about you by seizing your phone than they could if they obtained a warrant to search your entire home. I believe the law and/or the 4th amendment needs to be updated to clarify the procedures when it comes to seizing technology. The issue with technology is that you can never tell who was actually using the device at any specific time. Say you let your friend borrow your phone for a moment and they decided to text a friend about something illegal. Later that day you are pulled over and your phone is seized. The officer takes it back to his car and while he is looking through the information on the phone he sees the message containing the illegal act being talked about and automatically assumes you are the guilty party. With no evidence other than a short text message you are placed under arrest and taken to jail. How could a judge or jury rightfully convict a person on so little information? Obviously this is all personal opinion but I think it may be time for our legislative branch to take a look at this very real issue.
The answers to two of these questions are a matter of personal opinion. The answer to the third (your second question) is a relatively settled matter of law.
In order for a police officer to get a warrant to search or arrest someone, they need to have probable cause. They cannot simply search or arrest based on a vague suspicion. Probable cause is a higher standard than suspicion. It means that the officer has to have a reasonable basis to believe that a person is committing or has committed a crime.
If an officer has probable cause to suspect someone, the officer does not always need to get a warrant. The officer does not need to get a warrant if there is some sort of an urgent need to search and seize something. For example, if an officer sees you with a big bag of drugs, they do not need to go get a warrant because you could easily get rid of the evidence before they get back. So no, an officer who has a good reason (probable cause) to search you does not need a warrant.
The 4th Amendment is, in my view, certainly important. If we did not have this amendment, officers could search us and seize our belongings without any reason whatsoever. They could have checkpoints on the street where they just search everyone, trying to find evidence of illegal activity. They could randomly search people’s homes for the same reason. This would be a police state and I doubt any American thinks that would be a good thing.
As far as cell phones go, officers should certainly be able to search phones. However, they should have to have probable cause. They should be able to impound a cell phone if they think they have probable cause, but they should not be able to search it until a judge gives them a warrant.