Alternative to Altova's MissionKit
I was in the same situation as you are - designing service interfaces and doing a lot of XML stuff, but not getting the budget for buying Altova's XMLSpy stuff.
We settled for Liquid Technologies' Xml Studio - it has a free community edition to get you started and give you a feel of how it works, and it's quite extensive in supporting XML, XSLT, XML Schema (XSD) and quite a few more.
It's not quite as extensive as XmlSpy - but it does 90% of my job much easier than before. It's a stand-alone app and also plugs into Visual Studio.
All of the tasks that you mention are actually supported in XMLSpy Professional Edition, which is available for $499 (after the free trial. The XMLSpy edition comparison can be viewed here http://www.altova.com/xmlspy/edition-comparison.html