It is totally the cop's discretion which steps he takes before arresting someone for suspicion of DUI. Obviously, the better the case for probable cause the less likely it would be challenged in court.
There is no legal requirement for a cop to give FSTs or a PBT. Some people can't do FSTs for physical reasons, that refusal does not give rise to probable cause nor does it grant some privilege to then ask for a PBT.
Think of when there's an alcohol related accident. The driver is transported from the scene. That person obviously didn't do FSTs or a PBT. The cop then makes a request for a sample of blood drawn at the hospital, so long as it was drawn for a medical purpose. If a blood draw was not part of their treatment, a telephonic warrant to have blood drawn is needed.
A very small percentage of cases I handled actually had PBT readings. It's just an option to shore up probable cause.
Also consider that PBTs do not test for the presence of drugs. In a DUI drugs only case the PBT would show zero alcohol and the cop would have to rely on the behavior he witnessed and why his training led him to believe the driver was impaired. If the officer concluded he had probable cause he would ask the suspect for a blood sample and if they refused, get a telephonic warrant for the draw.
As to testifying about the odor of alcohol..... I've never handled a DUI based on alcohol where the cop said he DIDN'T smell alcohol. Saying there was an odor of alcohol is a gimme. There's no way you can go back and prove there was NO odor of alcohol. It's just like cops saying they smelled marijuana. Gives them reason to search a car. If you have nothing, no harm, no foul to a cop. If they find something it "proves" they were right.
(Kinda off topic: for a DUI drugs they don't even have to prove impairment. Just that a metabolite of the drug was in your system at the time you drove or were in actual physical control of the vehicle)
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