#1...I cut one room at a time using a $8.00 2" brush.
We use a good quality Dutch Boy or Behr, and I get the same results.
You cut and roll 1 wall at a time
This keeps a wet edge and allows the cut lines to blend into the roll
An $8 brush is not a quality brush
A premium line Wooster, Purdy, or Corona, will help
They will list closer to $20
I suspect your roller sleeves are also not up to snuff from the other information you have given us
Try some nice Purdy White Doves (which I know you can get) or some premium Wooster sleeves
These will list between $4 and $5
DB and B are not quality paints
You can minimize the effects buy switching your technique and using quality tools, but you are still just putting lipstick on some pigs
They are still pigs
Using a higher quality paint will raise you material costs slightly, but will speed up production and increase quality dramatically