Hey Kirby - I haven't seen your name in a while. Let's touch base sometime!
On a 10.6.0.2 box, the first request runs fine, whereas on a 10.5.0.11 box, I also get an internal error "*** Unknown command or macro". So, I suspect something is not altogether right on the 10.5 box, or maybe the new probe isn't ready on that FW, or I, too, am missing something. I issued the exact action XML on both appliances. It is interesting to note, too, the "CreateProbeCapture" action is in the "AnyActionElement" within the XSD of the 10.0.5.11 "xml-mgmt.xsd". Though you don't need the "<FilterByAPIClientID/>" at all, with our without it, it errors on a 10.5.0.11 box. The format of your SOAP Envelope is clearly correct, because it works exactly as typed on the 10.6.0.2 box.
On your 2nd call, your format is just a tiny bit off as the name of the MPG is an attribute. Consider this:
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
<soapenv:Body>
<dp:request xmlns:dp="http://www.datapower.com/schemas/management" domain="WTX">
<dp:modify-config>
<MultiProtocolGateway name="domain-status">
<DebugMode>on</DebugMode>
</MultiProtocolGateway>
</dp:modify-config>
</dp:request>
</soapenv:Body>
</soapenv:Envelope>