I can at least get you started on some of these:
Soul Reaver is a great 6th item pick on guardians if the enemy team has that ONE GOD who's ruining everything in team fights and you want to end him/her. It's also situationally viable on most mid-lane mages if you're having trouble securing kills or you're looking for more burst, but generally, the later you can put off buying the better off you'll be, and you usually won't buy it until you're looking to push for victory.
Osiris is still a very viable jungler, he just happens to excel in solo. Since it's popular now for HoG warrior to exist in the solo lane right now, you get all of the benefits from him without having to push through the early game. His early solo lane game is a lot cleaner than his early jungling game because of how ridiculous Death's Toll is on him.
3 & 4. There's not really a good way to concisely explain these, as a lot of it will depend on what the enemy team is doing versus what strategy you have. There are generally accepted strategies for both, but the way they subtly change in each match is what makes each match unique. If you're looking for general guidelines, try checking out the videos on Smite Guru's page.
Bakasura is sometimes used as a counterpick to Herc and Osiris, Nemesis counters Zhong Kui hard, Lassiz used to pick Bastet jungle to counter Vulcan mid/duo, and Ares is a good counter to Cognitive Prime.
Kumbha's root can be interrupted and isn't readily available.Threre's not really anything Kumbha can do that another Guardian can't do either longer (CC wise) or better (damage wise), but he's still viable. He actually makes a pretty dang good midlaner if you have a warrior support since his 1 can clear waves pretty early. Sobek has a phenomenal laning phase, but in team fights his only real peel is a displacement (his throw). It's a lot easier to mess up plays with Sobek than it is to set them up. That said, Sobek is about to get a nice buff in the next patch, and I expect we'll see a lot more of him. Regardless, here are Incon's videos on how to play them well:
Probably the best way to learn jungling is to watch the pros. I would personally recommend Lassiz, Andinster, Repikas, (all on Twitch) and the occasional DryBear youtube video when he plays a jungler.
I, personally, would recommend checking Tiermonster and SmiteStuff for popular item builds.
It'd be a good idea to grab AMC, Arachne, Baka, Chaac, Herc, Scylla, Vamana, and probably Neith since she's expected to be top tier after the other hunters get nerfed.