If there's such a burning inquisition and fanatical religion groups our there, I'm not sure if you can keep it all hidden enough. Still sounds like a lot of people would frown on it and as word would spread quickly about what you're done/become; closing a lot of (social/dilomatical/peaceful) doors and throwing you through more violent ones.
Okay, up until now you've been in a lot of hard and threatening fights, survived with some luck and gotten wounded... but how many more fights could you be thrown into again in the future, because of the necromancy, that perhaps could have been avoided if you hadn't taken the easy way out.
I understand he's getting paranoid, but how much more paranoid will he have to be when everyone has an actual legitimate reason to want him to burn because of what he's become?
That said, if you're set on going on with it... concealable necromancy?
If something which normally kills you suddenly leaves you alive and kicking, people are going to notice. A bone casing around the heart, lungs and/or organs so they're a lot harder to wound pr having them regenerate almost instantly could be well hidden, but it'll be noticed the second someone sticks a dagger in your back and you just turn around to see what happened there.
So I'm guessing things that help you avoid being wounded. Like SR's Improved Reflexes; vampires get more IP's too and are hyper fast in pretty much all vampire stories. Dodge quicker, attack faster; more damage out, less damage in, good all the way. Just keep yourself in check, never overdo it. If you're parrying and attacking with high speed and accuracy without taking it into extremes, you're just an extremely skilled fighter, "if I hadn't seen it for myself, I'd have said it was impossible!". but If you're dodging like the agents from The Matrix, it'll be obvious that you're not really human anymore.
How/why it works? Heightened senses on all fields, allowing you to feel the wind an enemies movement creates as he swings his sword, to hear the arrow swishing through the air, smell the enemy standing behind you... and a better muscular and nervous system I guess.
Hm, almost 2AM and out of whisky, time for bed.