I am no expert in this area but my understanding 're Sinemet is that the reason for your father’s non response could be that the dose/timing isn’t right for him yet as it can take some time to reach the right/best dose for the individual person. Alternatively, your father may have one of the atypical parkinson syndromes which do not respond to levodopa rather than PD. Many such people are initially diagnosed with Parkinson’s disease as the symptoms are difficult to differentiate, especially early on in the disease course.
Regarding DaT scan, there is a good, brief, easyread explanation of why they are not used routinely here https://www.michaeljfox.org/foundation/news-detail.php?ask-the-md-datscan-and-parkinson The situation is similar in the UK.
In your situation, if your father is not already being seen by a neurologist who is a specialist in movement disorders, ask for a referral to one for a second opinion - unfortuanately not all neurologists are equal. We did that a year ago after my husband was initially seen by a rather arrogant locum neurologist who seemed out of date, clearly did not have a clue and failed to make any meaningful diagnosis. My husband’s DaT scan result confirms that dopamine cells are not functioning properly and the pattern of cell loss indicates that the most likely diagnosis is PD. Only time will tell if this is the right diagnosis especially as his major symptom, amongst others, is unsteadiness/balance problems and he does not have a tremor.
Either way, you clearly need more answers and if your father’s consultant is a MDS, ask him outright how confident he is the PD diagnosis is right.
Best wishes & I hope you get some answers.