It's all a little outlandish, but the bottom line is that drugs work in unpredictable ways in some people, so some artistic license would be OK. My issue with Valium is that many, many people walk around under the influence of drugs in that class and are not obviously drugged. Still, if you increased the dose to say 10 mg three to four times per day you would certainly drug someone in the short term. As their body became accustomed to the drug, however, they would escape from this effect to a variable extent. On the other hand, this could be your plot device, with the same dose producing progressively less sedation over time and the person becoming more aware of what was going on.
If you do use Valium, you would need to crush the pills: they are not capsules. Also, at that dose and for that duration your character would definitely experience symptoms of drug withdrawal when the Valium was stopped: agitation, irritability, even frank hallucinations like alcohol withdrawal DTs. The drug needs to be tapered slowly to prevent this. That would be a problem for your plot, I suspect. Valium is also easily picked up in a standard urine drug screen test, if that matters to your plot.
There is no perfect drug for what you want, unless you invent one. As I noted, I prefer Haldol as the most plausible drug for your scenario.