I personally allow use of Perception to find obvious things - like "Oh look he has a bunch of big slash wounds." But I require Heal, as the general doctoring skill in Pathfinder, to make any definitive medical sense out of them, like "those slash wounds are/are not what killed him" (DC 15) or "those slash wounds were postmortem" (DC 20). Generally I'd just use the DC of a disease or poison as its level to be correctly identified as well, so a DC 25 poison is quite hard to detect. (It would be better if Heal had broken things up into diagnose and treat in the first place, but they didn't, obviously it's not a focus point of the system - you could easily expand the rules to have separate diagnose/treat DCs though). Perception should be a suboptimal choice here - it might give some evidence of symptoms but the player's on their own as to their significance. "He had a rash? Uh... Measles? Poison ivy? Bubonic plague? Who knows?"
If someone's tried to doctor up a body to conceal cause of death, it would be an opposed check - I had a group of PCs do this once. They had an NPC with them they meant to off, and they all got into a fight with a giant spider, then took the opportunity to kill him. Then they spider-fanged him etc. and did an opposed Heal (Bluff and Disguise assisting) vs the local priest they took the body to's Heal (Sense Motive assisting) to pass the guy off as having been killed by the spider instead of them. It worked!
If someone wanted to really be all House all the time (think Johnny Depp from Sleepy Hollow or even Samuel Le Bihan from Brotherhood of the Wolf) I'd make them take a more focused Profession skill (physician, forensics, detective, whatever). Heal should really only get you what decent medieval medicine would and not CSI level info.
Further evaluation of clues found would come under other skills. Heal could get you "killed by magic; there's weird burns on him that don't seem natural" (DC 15) but Knowledge: Arcana could then explain "it was magic missiles" (DC 20). Or Heal would get you "killed by poison" but probably require a more knowledge:relevant (nature, etc.) to figure out exactly what beast bears that poison.
In general this should be "really hard" - it's hard nowadays with modern medicine. "Died by poison via a big ol' injection mark; the clawing was postmortem" (DC 25) is a reasonable limit. "It was manticore poison"should be deduced or be a really, really high check DC (35).
I'll also note that if you really want to get into more forensic and detectivey Pathfinder games you should consider Pelgrane Press' Lorefinder (called Pathshoe when I playtested it), which adds more detailed investigation skills and approach from the GUMSHOE system into Pathfinder.
Cavalier is literally the "I am a Knight" class
Check it out in the PFSRD - it fills all the various Knight tropes, and isn't...insulting, the way some melee classes can be. Be warned that you may suffer from utility problems when it comes to out-of-combat encounters.
Knighthood confers no non-RP bonuses
Being knighted is an entirely roleplaying-based event and will not boost any of your character's ability scores or otherwise affect their sheet, especially not in Pathfinder Society.
Paladin Can Work, But...
I wouldn't suggest it. PFS can be unforgiving about alignment requirements, and it emphasizes Good over Law, which you've stated that you don't want to do.
The last character I played was an Elf Druid. She left home and started wandering as an adventurer because she never got along with her mother, and as she got older she got rebellious. The person who enabled her escape was a Human Druid who took a liking to her, and she wound up learning how to be a Druid from him after they eloped.
Did she care about nature, and want to protect the balance and all that? Sure (along with a dislike for large concentrations of people, like cities). But it wasn't what actually motivated her in the first place. Being a Druid is part of her, not her literal definition.
Druids Are People, Too
You're over thinking it. A Druid is a person. People have all kinds of motivations for doing things. Many of them have nothing at all to do with what their calling or career is. If you're not sure of "Why would a Druid leave and join an adventuring party?", instead try asking "Why would Bob leave and join an adventuring party?" When you answer that, then ask "Does that work as a Druid?" Most of the time, the answer is yes.
Off the top of my head, some possible answers:
A list for why a Druid might join an adventuring party is just as long as one for why pretty much any other class would join one. Some of those entries are "Druid-y" things, and some of those entries are just "people" things. You shouldn't limit yourself to just the stereotype that Druids are one dimensional tree huggers.