Is it possible to correct only the shape of nostrils without narrowing the alar base during an alarplasty?

It is difficult to comment on the best approach without seeing the exact orientation of your nasal ala and nasal width. That being said, it is not uncommon for patients to seek alarplasty following a rhinoplasty if they desire additional refinement to the nasal base and nasal ala. I agree that if it is the nostril margin and not the nostril width that you'd like to change, then alar rim grafts can potentially be placed to bring the nostril margin down. I'd suggest that you follow up with an experienced rhinoplasty surgeon to go over your exact concerns and have an in person examination. 

Hello! There are a couple ways to straighten the curvature of the nostrils. One does involve making an incision at the nostril base. You could perhaps consider something like an alar rim graft or other form of graft, depending on what your nasal anatomy is, to give a straighter line to the side of nostril. I would recommend a consultation with an experienced rhinoplasty surgeon for further recommendations; one really need to see and feel your nose to make recommendations specific to you. good luck!

It is difficult to comment on the status of your nostrils or the eventual surgical outcome without photos.  There are ways to straighten the nostril without narrowing the nostril base however the changes need to be performed conservatively to avoid nostril stenosis and/or scarring.