Nearly 50 years ago my father showed me how to fill a nail hole. It sounds simple but there is a technique to tit. I’m a firm believer that nail holes are to be filled using “Painter’s Putty”. They make it for just that reason. If you take a small wad of putty that fits comfortably in the palm of your hand, using your fingers, push the putty into the hole using your thumb, then using a semi stiff putty knife slide the putty knife between the putty and the wood. You achieve two functions by doing this...1) You are filling the hole all the way to the bottom and 2) if you cut the putty off with the putty knife while it’s still under pressure from you thumb the putty will have a small crown on top which when the putty cures will shrink to level of the substrate. Since putty is more dense than spackle it is less likely to crack or pop out. Think about it why do they make “Painter’s Putty and why do they call it a putty knife? In my opinion, spackle is used to fill imperfections in walls not for filling nail holes.
At the point where the crown molding meets the wall and ceiling and the corners, I would suggest a high quality NON silicone caulk, preferably a 55 year latex caulk. Keep a wet rag with you when caulking. The secret to a good caulking job is to keep your fingers clean of any residual caulk. If the cracks are on the larger size you might have to caulk it again the following day due to shrinkage, and it will shrink.
Best of Luck,
Hand Painted by Steve