1) WinRFA is not optimised for Win7 and noticeably unstable. Couldn't find a perfect compatibility mode as well, and the crashes buggered me as well, which is why I looked for alternatives. You might also be interested in trying BGA (beefed-up similar tool as winrfa) or direct batch approaches (look in the tool section and the Modding Tutorials "Debugger Multifunctionality".
2) Yes, you can change a lot ingame when using the debugger, trying out various values. To my knowledge there is no proper tutorial for this yet as well, but I have detailed this in the nightmap thread :
viewtopic.php?f=9&t=996#p6983 . In the second half of the post I describe how to use the ingame console to full effect.
If you want to change the values of an PCO, you first have to select that object by using ObjectTemplate.active XXX. Now every appropiate ObjectTemplate.x command like "ObjectTemplate.magsize" will affect the "active" object code.
All that is very useful and much faster than using the reboot/repack way when you just have to figure out values by trial& error.
Remember that everything you do and change inside the debugger console is temporary only and will not be saved to any rfa. So once you have figured out the values you like, note them down and change them in your source code and compile it to .rfa .