"Mader suggests that an average dosage should be 50mg/kg, a max dosage being 275mg. Our vet gave us 250mg pills, and this animal weighs 2.71kg. That works out to be about a 92.7mg/kg dosage. Is this too much? We are already keeping our eyes open for signs of overdose as described by Mader."
I would recommend crushing the pills thoroughly and making a liquid suspension just before administration, as many dry-to-wet preparations are not stable for more than a day or two after mixing. If your king is voluntarily taking food, you may place the crushed pill into a small segment of snake, or inject the liquid suspension into a prey item. If not, it's tubing time with the liquid suspension. I don't suggest putting a whole pill down a snake with GI problems.
If there is any way to get the animal to voluntarily take the drug, this is the best scenario for the patient. King cobras are what I consider a "species of special veterinary concern" due to the fact that extended physical restraint in these animals carries a higher than normal risk of patient injury and other negative consequences to the animal's health. Note that this has nothing to do with the risk to the handler; that's a separate issue.
100mg/kg in a single dose is still within a reasonable safety margin on Flagyl, but in most species I would rather see that dose split to 50mg/kg repeated every 2-3 days, or every 24 hours in a very severe case. In king cobras there is a legitimate argument to give 100mg/kg at a longer interval due to the negative health effects they suffer on veterinary restraint. When did your veterinarian recommend repeating the dose?
I use Diazepam at 0.15 mg/kg to 0.4 mg/kg IM for sedative purposes during veterinary procedures that require extended physical restraint on kings. Under prolonged conscious restraint, some really disturbing effects on a stressed patient can be noticed, specifically respiratory distress and cardiovascular irregularity. Our feeling is that for the patient's comfort and safety, sedation is definitely indicated to avoid these consequences. The lower end of the dose range is sufficient for rapid oral administration of medication; I use a higher dose (up to 0.8 mg/kg) as premedication for invasive or surgical procedures where the animal will be intubated and maintained on isoflurane and oxygen.
At 0.15 to 0.3 mg/kg you have a slower and calmer (but still quite dangerous) animal that will not go into respiratory distress during physical exam or medication. You should be prepared to monitor the animal for several hours of recovery in a safe environment (no waterbowl, no direct heat source they can overheat on, ambient temperature 80-82F). I wouldn't suggest going as high as 0.3 for this procedure; you want the low end of the range, 0.15 to 0.2 at the most. The sedative is for the patient's benefit, not the handler's, so you want to administer the absolute minimum that will keep the animal from going into respiratory distress during the procedure. Not zonk it out so that it is easy to handle; that can also have negative consequences to the patient's health.
If you are not seeing any stress related problems in your animal during physical restraint for exam and medication, sedatives are not indicated. An aggressive response to handling is fine as long as you aren't seeing respiratory problems on restraint (excessive drooling, wheezing and bubbling that looks and sounds like a nasty case of pneumonia) or stress related complications after handling (refusing food, behavioral changes). It's completely within acceptable health parameters for the patient if the snake charges confidently and tries to kill you. 
For that small problem you want a skilled handler, not sedatives. Keeping the animal's respiration and heart rate out of the danger zone during treatment is however a very good reason to use sedation.
Your vet may suggest acepromazine (worthless in cobras in our experience), Telazol (way too long a recovery with cardiovascular and respiratory depression), Ketamine (don't even go there on a king cobra) or some other drug he or she is fond of using for chemical restraint in reptiles. Unless your vet has extensive clinical experience with king cobras and can guarantee no adverse reaction in this species, stick with Diazepam.
If your vet would like to have a chat with another vet who has worked with this species, drop me an email (atheris a^t usa d0t net) and I'll pass on the clinic's information.
