Klonopin is in the class of drugs called benzodiazepines, as you are probably aware, and can be used in cats, though for your sized cat, often at the highest dose of 0.25mg per pound. Obviously your 9 lb kitty would only be prescribed at most 1mg, and your cat had 10 to 12mg.
The biggest side effect to this is sedation. Luckily this type of drug has an extremely high safety margin and it is quite difficult to overdose to toxic levels. Most dramatic side effects would've been seen that day: excessive sedation. However, some cats with high doses can become hypersensitive or aggressive. It is a muscle relaxant, and probably your cat had no choice but to just lie there that day! Luckily, almost a week later, your cat is obviously through the tough part.
As for lasting effects, the drug is indeed metabolized in the liver and is to be avoided in cats with severe liver failure. In cat's with normal liver function, chances are high that even this high dose, as long as not repeated excessively (which you're not), no problem will occur. You have a very high chance your cat is just fine. This amount of time afterwards, you would likely start to see signs if not: excessive urinating, perhaps lethargy, neurological signs.
The diarrhea and sneezing you are seeing now is likely due to some other infection going on, and a vet visit for that is recommended. Likely, he'll need antibiotics. Try a bland diet for now to help as well, like gerber baby food (chicken or turkey) and keeping him indoors, if he isn't already. Let him rest until he feels better!
Let me know if I can help further, and please click accept if this has answered your question. If not, just reply for further help on it.