-GameMaster-
→Symbol Ω Hunter←
I would max out Sceptile's HP instead of Speed. As you said, it's already quite fast, and if there's something fast enough to outrun it, as I said before, it will be an all-or-nothing situation since you'd take a lot more damage from it, or possibly get KO'd. Speed is more useful for competitive purposes.
Focus Sash doesn't exist in this generation. I don't know why people keep suggesting it lately.
The answer to this question is simple: I wouldn't have used one with an Adamant nature if I knew I would be using special attacks. I don't care if it's only one; it's still there, and it's still going to get used quite often. I would have used Naughty or Lonely instead, which would have lowered one of its defensive stats, which in my opinion is nowhere near as useful as its offensive stat. I would use something like 200 HP/156 Att/152 Sp.Att for EVs. A little less HP and more of one or both offensive stats could be considered. Really, it's your decision, not ours. You're the trainer, you need to be decisive about these things.
Do you think I should replace Flame Thrower with Blaze Kick then?