From your point of view it seems that the corrections are easily made to subtitles inside DVD SP, but it is not really so.
It is much more easier to make all corrections in Lemony and then bring in a new stl + graphics. Graphic-based subpictures are better suitable for subtitling in DVD SP than its built-in subtitling engine. They look more typographically correct, better rendered and leave no room for mistakes that can result from last-minute changes.
And yes, subtitling is much more complicated than people think. Subtitling should be always done professionally, this will save you a lot of trouble. Here's an old article that still relevant, 8 years after it was written.
http://joeclark.org/dvd/dvdsubs.htmlAlex Asp/Solaris Digital Ltd.