You can preview items on yourself just fine using ctrl+click, so...I really don't see the point of this post.
And yes, you can preview various pieces that aren't from the same set for a "mix and match" set. Just do what everyone else does and use CharPlan or ItemPreview for the links of the items if you don't have the actual aggs on you.
Also, it is stated somewhere that placing things in the wardrobe can't be undone. You should have read the fine print.