Hi Matthew,
1. When setting up a data connection to a SharePoint list, InfoPath uses the default.aspx view. If that view is limited to only display the first 100 items, that's all it will download. Obviously, you can't dynamically send a parameter to SharePoint to have it filter the view and provide a custom view when you go to query it. That's what WSSP does. It takes a parameter, calls SharePoint methods and returns a filtered list of items.
2. I think I answered this as part of #1. Basically, if there was a way to do this, we wouldn't have created WSSP. :) You can query all items in the ShP List/Library and then filter client side. No Web service or code required for that, but if you have a lot of data, the performance will be terrible and it won't scale. Once again, that's why we created WSSP.
3. Our DBXL Web Service Suite allows conditional submit to SharePoint library based on value in the form. You can have multiple form libraries (for example, for geographically disperse sites) and conditionally copy to those sites based on value in the form. Without DBXL, you could do this by writing code in your InfoPath form but you'd have to hardcode the SharePoint forms libraries and for each XSN form template, you'd have to copy the same code. Better to do it in the middle-tier, i.e. a web service. That's why we added that feature to DBXL v2.3 last year... :)
Lots and lots of features. You can go to our Qdabra.com site and click on Products and read more.