Hey OP and a belated welcome to Bluelight! I hope you're getting a lot of helpful information from the forums.
Now, in regards to your withdrawal arsenal, you really are better off than most. I'm assuming that you're going to be splitting your drugs 50/50? Going on that assumption, there are a few options for you. 15mg of Alprazolam (Xanax) is nothing to sneeze at. If you don't have a tolerance to Benzodiazepines, you will get a great deal of relief from these. It would be helpful to know how much Loperamide (Immodium) you have. I don't condone super-high-dose Loperamide, but you will definitely be needing more than the average person due to your Opioid tolerance. Costco has cheap Loperamide if that's an option for you. Also, how many Gabapentin (Neurontin) do you have? I don't mean to be a pessimist but 100mg is nothing. You will need much more to get relief. 2-3 grams is not an unheard of dose for Opioid withdrawal.
As far as your dope goes, I say do it now and get it over with. It's only going to be calling your name throughout the process, not to mention the fact that Opioid use during withdrawal will lead to a certain degree of backtracking to an earlier, more severe stage of withdrawal. Call it your last hit and be done with it.
Now, I'm assuming you don't have tolerance to Benzodiazepines, which of course is the ideal situation. I've been in a similar situation in the past, in that I've had to go through withdrawal but had only a limited supply of - in my case Clonazepam (Klonopin) - drugs to help get me through it. I had bud, Loperamide, Clonidine etc. All of these had a positive effect on my symptoms, but the Clonazepam is like a magive bullet in terms of symptom management. It enables sleep, decreases anxiety to a functional level, decreases palpitations and most importantly, it can totally arrest symptoms of Restless Legs Syndrome/Akathisia. The latter is the worst of the worst for me in terms of symptoms. So I was forced to ration my Clonazepam.
The question arose, should I take the Benzos before work, so I can function and not be miserable or do I take them before bed, so I might actually be able to get some sleep. It's a tough decision, because being at work and being sick is really terrible. The day drags on forever and generally your output is going to suffer because you're not 100%. On the other hand, if you don't get any sleep the previous evening, work can be just as grueling. It's really a matter of preference.
For me, it would potentially take 2mg of Alprazolam to get the symptom relief that I'm looking for. As chronic Opioid addicts, we can definitely acquire a habit super quick. Along with this ability to acquire tolerance and dependence at a crazy fast rate, the withdrawal from said dependence is just as terrible. But, you'e only been using for about a month. That's not that long, but I'm sure you'll be uncomfortable.
If I were you, I would try taking 1mg of Alprazolam before bed. Smoke some bud, take whatever sleeping meds you have and see if you can get to sleep. If you can't within an hour, I'd take the second milligram. You have a limited supply, so always go with the lowest dose possible. The Gabapentin is a little bit different. Again, if you lack tolerance, a large dose can last all day. I've had experiences last 24 hours in the past, so don't be so concerned about when you take it, but be aware that it takes a few hours for the effects to really kick in.
I would try to get some sleep the night before, do whatever you have to do. Wake up a little bit early, see how you feel. If you're so sick that it's really undeniable that something is wrong, then you cannot go to work. First impressions are everything. If you judge that you're too sick to function properly, then it's time to consider what cocktail of drugs you can take that might help you look and feel normal. Generally, it's going to be the same as the meds you take at night. Go easy on the Alprazolam at first. We are not good judges of our own intoxication, especially with Benzodiazepines.
You need to be the judge on what needs to be done to get through this. It's only a couple of days of really intense sickness. Keep telling yourself that. You've got some drugs that a lot of us would kill for if we were sick. Try to be responsible and not take all of your shit in one evening.