Thank you for that information. Let me address your questions in reverse order as that would make more sense.
The law allows one to be charged (read: charged, which is separate from conviction) if someone operates a motor vehicle while in an intoxicated condition. Police can use both direct and circumstantial evidence to charge someone. Direct evidence of a DWI is the typical pulling of someone when they are intoxicated either based on the officer's observation or based on the results of a breath/blood/urine test.
Circumstantial evidence is when the officers have evidence that, although not direct, would lead to a reasonable person to believe that someone was/is driving under the influence. Examples of circumstantial evidence include: statements of someone who was driving, but no longer is driving, but admits to driving one way or another (either directly to an officer or by filing a report immediately after an accident to someone who may believe that the person was under the influence of alcohol).
Another example of circumstantial evidence is if someone was driving a car, pulls into their house and goes inside. An officer then comes, feels the hood of the car is warm (which makes a reasonable person believe that the car had been recently operated) and the officer speaks to the operator and believes that the operator was under the influence.
Here, it looks like the police are relying on circumstantial evidence to charge your father. While circumstantial evidence is more than enough to charge someone, it would be a little harder to actually convict your father of DWI without other evidence.
2) Holding cell
Police are allowed to hold an individual after an arrest for a reasonable time. "Reasonableness" is judged based upon the need for the hold and applicable laws. One reasonable basis for holding someone is for processing. Oftentimes, the police are "busy" with other issues and cannot process someone right away. Another reason is a "DUI/DWI Hold." The hold is used to make sure that the arrestee sobers up, at least a little, before he or she is released. I must say that an hour hold on a DUI/DWI arrest, would be upheld as reasonable by almost every single Court for both processing requirements and sobering requirements.
Do you have any follow up questions based on what I have provided thus far?