Necro is definitely the main mastery and you could level with solo Necro as you said. Do you level as vitality skeletons then? That physical damage converted to vitality damage from Master of Death really annoys me.
I am pretty inexperienced with pet buids, but Demo should become useful when you start using Korvaak’s Burning-Blade and switch to fire pets. At that point, flame touch and temper should be great to buff the flat damage from your skeletons. The RR from Thermite Mine and BWC also become quite important when playing fire pets. The thing that annoys me the most about this build is that Master of Death ruins your physical to fire conversion, so you are probably better off without an exclusive skill.
I came up with this sketch (with a lot of ideas taken from gargabolo’s and rekt’s fire skeletons build, and your budget necro from Carnival) for a budget fire skeletons defiler: https://www.grimtools.com/calc/gZwordbZ. Considering it has zero affixes on the MIs, it looks like a decent budget starter. I am just not sure how the damage/survivability will be since I am inexperienced with pet builds. Also not sure about my devotions. I took Bysmiel Bonds because I didn’t think another 3 green devotion and some extra points in Ulo would be worth it, but I might be wrong. I have two sources of flat RR (with the exact same value btw), which is also quite questionable.