Yes, it should work, but you probably will experience some lags considering how old the 3GS is.
The update 6.1.6 helps to maintain the working efficiency for the iphone 3gs.
Considering updating your iPhone forces you to update to the latest software available for the device, updating an iPhone 3GS now will update it to iOS 6.
yes
Yes.
all the features that are mentioned is available with the iPhone 3gs
Yes it does.
Yes
No. The iPhone 3GS is sadly a GSM only phone, so it cannot function on Sprint (or Verizon's) bandwidths.
depends what you want to use it for if it's for business i would recomend the blackberry but if it's just for a phone i would recommend the iphone3GS I think the iphone 3G is cheaper than the iphone 3GS and you can get video on the iphone 3G not just on the 3GS however the iphone will not work as well as it in America is done by at&t which do not cover most of the country but will cover most of the east coast
I tried it with my 3GS and it does work.
Probably not, they have different exterior shapes.
iPhone 3gs