Round nostrils, would tip sutures change nostril shape to almonds and hold up over long-term?

It i impossible to give an opinion without photos. The appearance of your nostrils is the result of several different aspects of your nose. The size and support of the tip, the height of the dorsum, and width of the base of the nose are the most major determinants. The thickness you describe would also make a difference. Perhaps you should go get a consult. Good luck.

Suture techniques of the nasal tip are performed along with a conservative cartilage removal when patients have a wide and bulbous nasal tip cartilages. Suture techniques are not used to change the shape of the nostrils. Thick skin will be a significant impediment to seeing very refined results in the tip of the nose. For more information about bulbous tip reduction, please see the link and the video below

This is a thoughtful question. The short answer is yes tip sutures can change the shape of the nostrils. The problem with tip sutures alone is over the long term. Most surgeons are now leaning towards cartilage grafting the nasal tip and alar (side rim of nose) areas if changes are sought after. 

The cartilage will last forever. It is your own cartilage and typically taken from the nasal septum, ear or rib. It makes sense if you think about it. Cartilage and the skin soft tissue is what gives your nose shape right now, so when you want this changed we try and do it as natural as possible. 

Having medium thick skin that is oily is good and bad. It is good because it allows a smooth result with less imperfections post operatively. It is bad because there is typically more swelling post operatively which will increase the time until you are happy with your results. 

Hope this helps.

DrC