Yes, you can use regular retouch varnish with wso. Water Soluble Oil is regular oil paint modified to mix with water so they actually behave like regular oils once any water has evaporated from them.
Oiling out is using a medium but you use very, very little of it and it dries fairly rapidly. I've done it before and had good results. I used the WS linseed oil on a touch dry painting - the painting must be dry to the touch or you risk pulling up paint layers.
To oil out, take a soft, lint-free cloth (a strip from an old t-shirt works well) and dip it into your linseed oil. Be very frugal with the oil and don't pool it onto your painting. Wipe the oil onto the painting and work it in small circles over the painting paying particular attention to the "sunken" or flat-looking areas.
There should be no areas that are shinier than others as that means you've used too much oil in that area. If you have any such areas, go over them again with a dryer cloth.
It's simple and effective and will not harm your painting as long as the painting is touch dry. Let us know how it works for you if you try it.
Hey, congratulations on being commissioned for a portrait then. Whether a show or an unveiling, you're still getting public exposure and that's all good! Can't wait to see the painting.