1. Bringing someone along for poisona/haste isn't solo.
2. /RDM fast cast gives you 10% casting time and 5 % recasting time. To me, this is just as valuable to nin/rdm if not more than the magic attack bonus. Stack on Swift, Fuma's, maybe even warlock mantle and you should be able to ni to ni pretty easily. Also, the lowered recast on elemental ninjutsu helps so you don't have any pause between casts (i.e. if you are casting them one right after the other, by the time you get to the last elemental ninjutsu, the 1st one you cast is not usually ready to recast unless you are /rdm). Its pretty neglible, but it'll add up over time, and the faster you kill the less room you have to make mistakes.
3. The benefits of /drk aren't really that good. Arcane circle only lasts for 1 minute with a 10 minute recast timer. That means you can get 15% bonus for 2 minutes max, or you can /rdm and get MAB+20 for the whole time. The activation rate of Arcana Killer isn't static, so it won't always be 5% (it will of course average to 5% over time, but if you are counting on paralyze/intimidation procs to keep you alive, you are essentially just relying on luck to keep you alive). The only spell that being /drk gives you at 50 that you can use, is Bio, whereas /rdm would give you Cures, Bio/Dia, Aquaveil, Pro/Shell, Blaze Spikes (meh), en-spells (mostly meh), Regen and a variety of barspells. /Drk does give you last resort, which I guess it good but it doesn't make up the difference for the other benefits /rdm gives you.
4. There is absolutely no reason to /thf for this bcnm at all. All that /thf gives you is 10 evasion and flee. If you need the evasion bonus, /dnc gets it at 15 and would be WAY more useful than /thf.