There are no restriction on foreigners marrying other foreigners either offshore or onshore (but ROM might be hesitant to issue a marriage certificate here if they know about her previous WP though). However, should you get married offshore there are no real restrictions that I'm aware of unless it's changed in the past year or two. Your understanding is okay, SC's & PR's have to get permission to marry a WP holder or even a long since gone former WP holder, even if she's been gone 20 years.
The only other problem is that if it's a new offshore marriage and they find out she was formerly a WP holder, you may well find out the path to PR or SC may hit a snag that they will never tell you about, but you will have no real problem with an EP and a Dependent pass if legally married, but they could be nasty if you got PR first, went overseas, got married and they find out she was a WP, you might have a very hard time even getting her a LTVP.
Good Luck and yeah, you got the makin's for a good soap!