We're all guilty about it. --> You have actually combined two phrases to make an unnatural one.
"We all feel guilty about it" --> Perhaps they have not done anything but feel guilt.
"We're all guilty of it" --> They are admitting to doing a bad thing.
I have never seen you acting like this before. --> This is grammatically perfect but I think using "act" not "acting" makes it a little more natural.
We are in the preparation process for the new season. --> This is grammatically perfect but it seems to wordy because "prepare" is more commonly used as a verb, so the following sounds more natural:
"We are preparing for the new season"