Hi
@bdalton, thanks for your questions. I'm no expert but I'll try to answer as best I can.
1. this question has almost become a political one, and some people are really opinionated on how "close" they are to each other. I think they're just considered different dialects, which share the same grammar structure and written system, but are pronounced according to their own rules. They are written using the same characters, but mainland China (which uses putonghua or "standard" mandarin) will use the simplified set, while Hong Kong (which uses guangdonghua or Cantonese) still uses the traditional set, for the most part. Meanwhile Taiwan, who uses a traditional set of hanzi, also speaks mandarin. You can read simplified and traditional hanzi in either dialect, so I don't think it matters so much.
2. Modern Chinese is more commonly found written horizontally, left to right. As for breaking the phrase up, I think you just want to avoid splitting compound words like 戰士 and 戰場. So I guess, somewhere near the middle? like:
| Simplified | Traditional |
花园里的战士好过
战场上的园丁 | 花園裡的戰士好過
戰場上的園丁 |
I don't think you'd put 好过 at the beginning of a line, but my Chinese isn't good.