You’ll not find any decisions about FP’s future on this forum as it is a community forum and not a place where the FP team discusses their decisions.
You can contact them directly, but I don’t think they will leak anything before they are ready to announce it to the public.
Yes.
It also depends a lot on your screen brightness, how many/which apps you installed, which games you are playing and whether you are browsing news articles or high def youtube videos.
There are some limitations to the dual SIM feature and not all functions will work in all countries (different frequencies - more info here), but in general: yes.
Fairphone announced that they are 99% certain that they will upgrade to a newer Android version. Last we heard they were looking into whether upgrading to Android 7 right away is better or whether Android 6 is more realistic (for now).
Porting different Operating Systems is a community effort, so if you have something to contribute I’m sure it’s appreciated. See here.
Since FP just announced new covers and are working on an upgrade to the camera module I’m very certain that - if they are even working on an FP3 - it will be a very long time until they are ready to announce it.
I have no experiences with Samsung phones - I don’t think I ever had one in my hands. If look at it just from the financial side: If you get a used phone for e.g. a 5th of the price of a new Fairphone then it would only have to last 1 year compared to a FP you use for 5 years. Also reusing discarded but functional phones is always better for the environment than getting even the fairest phone there is.
On the other hand if you want to have a device that you can use, love and feel good about for a long time and join a cool community then a Fairphone is a great decision. 