Is it possible to hide/show bracketed chords song based and not as a database option?
No, this setting will remain until you change it, so you would have to switch it on and off as you move through your songs. I think it would be more confusing if this were set at the song level, and there might be a better solution for whatever you're wanting to do here.
This would also be very useful for the “transpose recordings” function, because I use some recordings several times in different databases and different tunings/pitches.
In that case, especially if you are using the recordings in a performance, I'd suggest pitch-shifting the recordings in an audio editor and then attaching multiple versions of the recordings in BandHelper. You will probably get better quality than you'd get from the built-in pitch shifting, which is meant more for practicing.
Is it possible to use the function “two columns for lyrics or documents” also song based and not as a Layout option because some lyrics in a database are perfect for it, others are not?
No, sorry, this is set at the layout level. But I could see the value of changing that at the song level and I'll put that on my wish list.