1. The "positive findings" were cocaine, levamisole, carfentanyl, U47700, nicotine, and diphenhydramine.
2. Does it matter? He could have taken some while out, some at home, or all at home, or all when he was out, but a mixture of the two seems likely. The problem with toxicology reports is that all it can tell you is the amount present in blood/fluids at the time of death, it can't say if he took a small dose recently or a larger dose a long time ago, because those produce about the same amount of drug in tissues.
3. Impossible to say, if there's no dissolved residue from pills in his stomach (which would have to have been consumed very shortly prior to death, like less than 10-20 mins), no major nose irritation, and no fresh needle marks then you can't say for sure.
Death was likely from respiratory depression caused by the opioids carfentanil and U47700, potentiated by diphenhydramine. It's likely that he was doing cocaine at the same time, which was keeping him stimulated and awake, but then once the cocaine wore off he lost consciousness and stopped breathing because the opioids were just too strong.
The shower wasn't really to blame.
If you are going to search this person's personal effects and their living space, please do so with utmost care, or let the police do it, you don't want to inadvertently expose yourself to carfentanil.