Is it safe to cut off from the inside of my helmet?
Unbeknownst to many consumers, different helmet manufacturers use different molds for their helmets. Just like some shoe brands are known to be thinner or wider than other brands, some helmets are more oval, some more round, and some pointier. Even from the same manufacturer, their racing models might be different from MTB, skateboarding type, and general lines. And there might be a helmet with a vent hole in exactly the right place out there. 

It's important to find a helmet that fits correctly and comfortable. It might take having to go to different stores and trying different brands and types (or even to a store in a different country, if you have the opportunity). 

That being said, through, if it were my head and I had to decide between a helmet that was partially shaved to fit -- or a larger helmet with some extra padding to fit; I would choose the latter. The helmet manufacturers provide extra padding pads of varying thickness in the box because they know they can't produce a helmet that fits perfectly on everyone's head. As long as the helmet is snug on your head and not uncomfortable, you should be good to go. Remember that many cyclists wear beanies in the winter and cycling caps in the summer under their helmets -- something that is well known to the helmet manufacturers.

Remember the worst helmet is the helmet you don't wear.