Try pouring some really hot water into it (not boiling unless you know the plastic can handle it) and give it a good shake, then pour it into a wide glass bowl. Look closely at the surface for any traces of oil. If there is none on there then there is probably not enough residue to be a problem. If you are still worried, use warm water and soap and shake the hell out of it until your arms hurt. Then get someone else to shake it more, rinse and rinse again. Anything left after all that is probably not going to suddenly jump into your cool, soap-free beer.
I bottle from a pressure barrel. You can use it as a secondary FV if you like, or just batch prime into it. I connect my bottling stick to the tap via a short length of plastic syphon tubing. Needs a bit of boiling water to make it soft enough to go over the pb tap at first but after a few uses it stays the right size.
It allows me to bottle the vast bulk of my beer without requiring an assistant. The last bottle or two can be a little tricky, but for me the alternative is all my beer in pressure barrels and I just prefer it bottled, despite the hassle.