I tested your suggestions this morning.
Thank you for trying the suggestions! Let me assist you with your questions.
1) If the tree was selected in the "Objects to Scatter" list, and right-clicking didn't work, it might be something particular to the tree that the program didn't predict. Is is the tree a prop or a figure? I made the tool as generic as possible to accommodate for all sorts of props and figures, but they are not all built the same. I might have to test with that prop or figure to see what's happening.
2) The "origin" is called "Center Point" in the joint editor. Once you select the figure's "Body", it usually help to set the center point to zero in X, Y and Z, and then the "end point" to zero in X and Z, and then set Y to something like "0.1". This last one (setting end point Y) is very important - it should always be a positive value larger than zero. Orientation should not be changed. For the record, the scatter tool automatically zeroes the origin point for you on the scatter objects it creates, but it doesn't change the end-point (except for billboards). This was the best generic way I could come up with, but I cannot predict the creative ways content creators build their contents. Poser is an incredibly varied market, and there might be props and figures that break the rules somewhere. For example, if a prop or figure was not modeled at the origin (and above the ground), there is nothing the tool can do to fix that.
What you can do, however, is to convert the sheep from figure to prop. Just make sure it's on the ground (CTRL+D) when you do it, and placed at the origin (center of the scene). There are many ways to do that in Poser. I usually do this using Dimention3D's "Figure to Prop" free script, but I don't know if this is still available. Another way is using Poser built-in Grouping Tool, but that might need a specific tutorial. In a nutshell, you create a new group, select all, and press the "create prop" button. It's the same process I have demonstrated in the last video tutorial posted here in this thread, where I have created 2 props out of the Poser ground using the Group Editor. That can be used to convert figures to prop. When we do that, it preserves all the materials, so the object will still look the same, and it fixes whatever issues it might have before with the scatter tool.
3) I have tried scattering the HW horse, and, surprise, it really can't be removed with the Scatter Tool. One would assume the code to remove one figure should work with any other, but here is the proof that not all is the same under the sky. I would convert the horse to a prop (after posed and textured), and that should resolve any issues like explained above. Chances are it will also be a little faster. Give it a try, and ask me if you have questions.