Is it possible to use "unlock with fingerprint" feature, and be asked for (after boot) giving passphrase and fingerprint on encrypted Galaxy S6
I found myself an answer to part one of the question: Yes, on the Galaxy S6 you can apparently use the fingerprint sensor to unlock the device after encrytion. 

Infomation found here:

http://forum.xda-developers.com/galaxy-s6/help/fingerprint-unlock-device-encryption-t3081102

Has somebody something to say for part two? :-)

Edit 2015-10-06:
I got my S6 last week. Here is the story:


Encrypting the device and using fingerprint unlock does indeed work. :-)
When configuring fingerprints you have to set a backup password.
The backup password is used to encrypt the device's encryption key. --> Whenever you reboot the phone you must enter it. You cannot boot the phone using fingerprints alone.
After a few unsuccessful attempts to unlock the device using the fingerprint sensor the device forces you to enter the backup password. 
I could not find some kind of timer to configure. If you want to force the input of the password you have to power down the device or use an unconfigured finger a few times in a row.


This is a near-perfect solution for me (except for the timer issue, but I can live without...).

Hope this information helps somebody!