Hey!
1. Serotonin Syndrome is a very serious, potentially deadly medical condition. It is always mentioned as a matter of due diligence, though in real life, Serotonin Syndrome is a rarer condition than you would expect considering all of the discussion about it.
With that in mind, I'm gonna tell you the same thing. Dextromethorphan (DXM) taken with SSRI's could cause Serotonin Syndrome. If you're using reasonably responsible dosages of DXM, it is not likely that this combination would produce Serotonin Syndrome. I will say, there will be no doubt whatsoever that something is very wrong with you if Serotonin Syndrome comes on. You're got gonna be sitting on the couch eating potato chips wondering if you have Serotonin Syndrome.
It's just good HR to talk about Serotonin Syndrome, but it's not something I would lose sleep over worrying. If you ever have concerns, go to the hospital.
2. I take 100mg Fluvoxamine per day with 70mg Methadone. I've never had to increase the SSRI or seen any reason to. So I believe the short answer here is no.
3. You would be taking it every day to get your levels stable, so the timing is not super-relevant. I suppose taking the Fluvoxamine ~5 hours before taking the Methadone would provide the most efficient results, though I believe the timing is pretty negligible.
4. I am not familiar with any cases of fatality caused solely by the combination of Methadone/Fluvoxamine. The case I posted earlier about the female being admitted to the hospital illustrates how this person was still able to seek help for themselves. If you're injecting Fentanyl, it's pretty hard to Naloxone (Narcan) yourself. You should have ample warning regarding your health due to the long-acting nature of the SSRI.
5. I still take Fluvoxamine and Methadone. I take 100mg Fluvoxamine and 70mg Methadone. I've been on doses as high as 150 in the past. To illustrate, I feel noticeably better taking the smaller dose with Fluvoxamine than I did simply being at the higher dose. It did not just make the Methadone stronger, it made it "better". Now I would never take Methadone without Fluvoxamine, as the Fluvoxamine is ultimately what allowed me to be satisfied with Methadone.
6. SSRI's are pretty safe and well-tolerated. The most common complaints concern reduced libido/sex drive. I'm not familiar with any serious chronic complications. You can potentially get the benefit you're after from a low(er) dose like 25mg. At lower dosages, the libido issues are less prominent.
7. Yes, but not in the same way Opioids would. SSRI withdrawal generally consists of mild anxiety and confusion for about a week. I've been through SSRI withdrawal, it's nothing if you've done something like Opioids; a minor inconvenience. Even still, you can avoid the withdrawal syndrome by doing a short taper. So the answer to your question is that dependence/tolerance to the SSRI is not anything to worry too much about.
I have also read that Fluvoxamine stays in your system for 3 to 5 days. If I took it daily my blood serum level would keep on increasing until it reaches a dangerously high level, correct?
Nope. You will reach a steady state following about a week of administration. It doesn't work like how you've described so don't worry about that.