Hi,
1. What I mean is that to call the URL of the shipstation plugin it looks like that:
http://localhost/Joomla%203.3.3/index.php?option=com_hikashop&ctrl=shipstation&SS-UserName=philippe&SS-Password=domoshima&action=export&test=1&start_date=10%2f8%2f2015+12%3a47&end_date=11%2f26%2f2015+18%3a55&page=1
You can see there a start_date and a end_date parameters. So the URL will give you a XML of the orders between these two dates.
2. You can see the format is a URL encoded date format like that: 10/8/2015 12:47
If you want another format, you can use any date format supported by strtotime:
php.net/manual/fr/function.strtotime.php
3. Yes I do.
4. It depends what you need but it doesn't look difficult. A seasoned PHP developer shouldn't have much problem doing it.