Ok, i have to take issue with a few of these claims. 1. (from the wikipedia entry on mitragynine) Mitragynine itself acts primarily via μ-opioid receptors, though its oxidation product mitragynine pseudoindoxyl, acts as an even more potent and selective μ-opioid agonist with less affinity for δ or κ receptors.[2][3] Another alkaloid with a major contribution to the μ-opioid activity of the kratom plant is the related compound 7-hydroxymitragynine, which while present in the plant in much smaller quantities than mitragynine, is a much more potent μ-opioid agonist.[4] - so to say kratom works by "some crazy receptor group" is COMPLETELY false. Just because of the tryptamine structure, make NO mistake it is indeed a μ-opioid agonist. and 2. That you can only cause PW by taking suboxone AFTER a full agonist.
Feel me out here, OP reported injection a kindof INSANE ammount of bupe. The first bout of PW was quite likely from using it over the kratom, which as we have just learned is indeed an opiate agonist. He IV'd at LEAST 12mg of bupe with a tolerance where kratom is still useful. I'd wager there was quite a bit left in his system a few days later, just not enough to fully blockade the fentanyl effects. However, once the short half life of the fent moved it out, the suboxone would be coming back full force, effectively making it like "taking bupe AFTER an opiate agonist" just my .02 and wanted to clear up the misconception about kratom.
This does not sound like PW as far as I am concerned, I really sincerely doubt that is the issue here. For one, that day the OP had already IV'd multiple, large doses shots of Suboxone. If he was going to get PW's it would've happened on the first shot, not he 5th shot he took 12 hours or so after the 1st one. Also you don't get PW's from taking a full agonist with Subs already in your system and then the full agonist wearing off. I have done heroin a few hours after taking Subs and after the H wears off I am just right back where I was before the heroin. All that happens when you do that is some of the full agonist is blocked by the bupe, depending on how much is there. If there is bupe in your system its going to be on a receptor, it beats out almost any other chemical that acts on opiate receptors. If the full agonist had receptors to act on, after its gone those receptors are just empty, all the bupe is already committed to other receptors.
Even if it made sense that this was PW's from the events that transpired, just read what his symptoms were. Those don't sound like PWs, all PW happens to be is a real intense w/d. He would be shitting his brains out, having really bad kicks, pooring sweat, very nauseous and probably vomitting.
Now cotton fever makes more sense in terms of how he felt. Now cotton fever would have made plenty of sense when it happened the first time. But when it happened that second time it seems odd. Is there any incidences of someone feeling the effects of cotton fever return a few days later? Might be worth looking up. As I see it, though, considering the situation its worth noting the possibility of some other, more serious ramifications of IV drug use.
Wish we could get a follow up from the OP on how things have been going, hope hes doing ok and all.