You can edit the SEP profile and adjust it until it's reliable enough. The thing about SEP is that it requires fine-tuning for some setups and you have a GUI editor to select the parameters, it's trial and error to get the results you want. If you have a profile which works consistently, please share it and let us know so we can check it out.
Good question. It is possible, go to Settings --> Configure KStars --> FITS , and you can see HFR SEP Profile. Select one then you open FITS image --> Mark Stars.
Then you can go back and "edit" the select SEP profile and change settings as you see fit.