I don't know about Windows Live, but I am afraid most sites won't import those keywords as they are in XMP as you said, not IPTC.
But luckily there are free programs for IPTC: Irfanview, XnView and many more.
Regarding your other questions, there are many IPTC fields that are unused, you may add your comments there instead as keywords. But it's easier to keep track in a spreadsheet, or upload a batch to all sites at the same time and then move to an "Uploaded" folder.
Actually some sites prefer XMP it seems. I include the metadata in both formats to make sure.
XMP is the new XML-based "Extensible Metadata Platform" developed by Adobe in 2001. Adobe worked with the IPTC to incorporate the old "IPTC headers" into the new XMP framework and in 2005 the "IPTC Core Schema for XMP" specification was released. XMP is an open-source, public standard, making it easier for developers to adopt the specification in third-party software.Of course Microstock may be behind the times and not up to 2001 standards?
