Width of Memo in 2 line display

You aren't talking to the developers here so anything "answer" to your question will be speculation.
But as far as I know the register is a highly customized/complicated set of controls which is decades old.
It is extremely hard for them to even have the 2 line mode let alone have it "fully customizable".

If you look at Quicken Mac which is a newer rewrite you will see that they decided not to even provide a two line mode.  This allows them to the standard Mac window APIs and it is much more customizable as far as order of columns and such go.  And they say that it is much more maintainable then the code that it replaced that had a two line mode.