Two or three of these I can try to address...
3. The sounds are REALLY bad. Can't you, for example, record stuff from movies? Like the shrieks from Aliens series or whatever?
...And scream when wounded too?
The project has to use only either original content or content that is freely licensed under Creative Commons or GPL of certain types, because of copyright issues - Most commercial movies, films, and music are protected by copyright and therefore stealing content from them would infringe on the rights of those who created such content, leading to potential litigation. This project is distributed freely, and one can not rightfully (legally) pass around something they don't have rights to.
If you are willing to create your own original works that you think would be better or would fill in the missing sounds, as far as I know anyone can offer a proposed contribution (as long as, again, they are original and not derived from copyrighted works). If you have any programming experience you could also offer to help code some of the things on the TODO list (Sorry I don't have a link at this moment, one of the devs could help you with that.)
6. I'm not sure, am I supposed to actually click on reaction fire button to activate the mode?
I believe the button toggles that, yes, and if there are enough Time Units left you can also set reaction fire for multiple shots, if I'm not mistaken.