You can start a new line with Option-Return; the cursor has to be in the text (i.e. don’t have the whole cell selected), but that would normally be the case if you’re typing in the cell… You can set Return to make a new line in OO preferences
By “bullets within a cell”, if you mean write a bulleted list in a cell, I don’t believe there’s a way of doing that. I believe the designed behaviour is to bullet the cell and make a list that way.
On your first point - the way to do that would be by applying styles. The manual is pretty good on how to do that