What better way than trying it out. I was sure of what I've to say, but I did try out scenarios you mentioned. Details below:
The People App in Windows 8 does list Contacts from all of your accounts and shows information combined from Google, Facebook and other connected accounts.
However information from sources remain independent. They are not updated or modified without your explicit consent.
Coming directly to your questions,
1) Your Facebook contacts and Google contacts will remain independent and will not show up in other services. I recently installed Windows 8 Pro and connected Facebook, Google, Outlook and few other accounts. My Google contacts page continues to show only my previous GMail contacts and Facebook contacts are not listed.
2) Your Google contacts will remain as they are. No details (including the photo) would be changed just because you connected your Facebook account to the People App on Windows 8.
3) For editing any contact, you need to explicitly specify to which account the changes must take effect:
Of-course you would not be able to edit the contact's Facebook info, but you can choose between your GMail and Outlook accounts.
I edited the Work address and Company information for a contact (after choosing GMail). The information immediately showed up in my Google Contacts (accessed thru Web).
So its absolutely safe to connect all of your accounts in Windows 8. It would enhance the experience and at the same time, cause no trouble!