Maybe cornstarch and baking soda your soda too? I've done baking soda on mattresses and it helps. You'll need aLOT of baking soda though - not from supermarket, more like B&Q if they sell it.
The showering shouldn't be up for discussion. Your DH needs to be tough on this. If they don't want soap then Sanex is easily available and doesn't have soap/parabens/scent etc in and isn't expensive. Other more expensive options obviously available too.
He needs to sit them down, tell them he loves them, they are getting older now and need to take care of themselves. That when they're with him they need to arrive, shower and change. They're not babies, hormones produce odours and it's socially unacceptable to smell. He's going to have to spell this out to them, because they actually don't know. Reiterate he loves them, he wants the best for them, and that they're not children now, but young adults. As such, in his home (I get it's yours, but using "his" to be clear it's from him) they're going to have to behave like it. Shower upon arrival after rugby and then every morning or evening - they choose).
Don't add threats. If they don't do it, then wifi off threat. If they still don't, turn it off. And take phones if they have 3-4G.
It needs to be non-negotiable, because it is non-negotiable!
I feel really sorry for them.
I also can't bring my mind to think about having periods (assuming DSD has started) and only washing once a week. What mother could think that was ok for her daughter?!
If none of it works, take them swimming! (Kidding - they'd need a pre-pool wash!).