from object-creating tools. You can paint anything any color in any tool, so how would you decide which tool has to remember this color as "last set"? By checking which tool was active at the moment of style setting? But then the majority of "last used styles" will be in
No, the other way round. When you use (not select) an object-creating tool, you update its remembered style to the current style. Then, when you select that tool, you set the current style to the tool's remembered style. It would work just the same as the existing "one style per tool" setting, but Inkscape changes the one style when you use the tool, and the setting is stored with the document metadata rather than in global prefs.
a rect and an ellipse simultaneously? As you see, it's not simple and obvious either way, and I'm afraid it will be just too confusing for the users.
It's not simple or obvious, but if you assume your users are that easily confused, you'll never produce anything more challenging than an Etch-a-Sketch.