this is what I do in situations like this.
Firstly, the Query List action will only return one field. That is all it is built to return. If you needed multiple fields, you'd have to have multiple Query List actions. So that already helps.
Next, what I do is open the Query List action and select a list that is similar to the one I'm going to query. If you are going to be querying another site, just for this step, go to that site, go to the list and start creating a workflow. (you won't be publishing it or saving it). Add a Query List action, select your list and then you'll be able to configure the filter part.
Once you have done this, click the CAML Editor check box, and you'll see the CAML that has been built for you. You can now copy this CAML (xml) and go back to your real workflow, open your Query List action and in the CAML Editor mode, paste in your CAML query.
The hard work is done. Now you just need to make some small modifications to the filter part to make sure you are filtering based on information you currently have in your workflow. eg. you may need to insert a workflow variable or a common property.
I hope this helps.