Hello kivi, that's great!
1. The username and password is changed in traditional linux-style by editing the configuration file /etc/pellmon/pellmon.conf. The file is a "text only" file that can be edited with any text editor, it's quite long but it contains lots of explaining comments. You need to have root rights to be able to save it in the correct place, so you can either save a copy in your home folder and then use a terminal command to copy it to the correct place: "sudo cp /home/pi/pellmon.conf /etc/pellmon/pellmon.conf", or you can start the editor with 'sudo' to give the editor permission to edit the file in place.
To do everything in a terminal, which is very handy if you have no monitor connected, you can for instance edit the file directly with 'sudo nano /etc/pellmon/pellmon.conf'. (Nano is a simple text editor that runs in a terminal)
Look for this section:
[authentication]
# username = password, for webinterface
testuser = 12345
The rows starting with '#' are comments, the default user is 'testuser' and the default password is '12345'
To activate a new configuration just reboot, or if that feels too drastic you can instead restart the pellmon daemons with these commands:"sudo service pellmonsrv restart", and "sudo service pellmonweb restart".
2. Regarding the timers for VVB and heating, there is apparently a bug here... The relevant parameters are found in the meny 'Timers' but they are hidden until you click the menu selection [More...]. There are a lot of seldom used parameters in a scotte and this More.../Less... thing is supposed to show/hide the less useful ones, but the timers should definitely be in the always shown selection! The more/less distinction is btw very hastily made so improvement proposals are gratefully received!
Hope this helps!