This is going to make some of you upset, and for that I apologize in advance. But you need to know the hard truth now.
If it bothers you that your figures may not last a lifetime, sell them right now.
There is no spray, powder, humidity level, light level or temperature that will stop the breakdown of rubber, plastic, wood, metal or cloth. It doesn't exist. I will repeat that for those of you who didn't hear me. YOU CANNOT STOP THE DETERIORATION OF PLASTIC AND RUBBER WITH ANY KNOWN PROCESS OR ENVIRONMENTAL CONDITION. You can possibly slow down the process, but probably not unless you have a museum type budget for a custom display case. All the tape and cups of water in the world won't solve your problem.
I worked for ten years in a museum, curating and cataloging a collection of rare documents and artifacts. I have been involved in climate control conferences, talks on molecular stability of chemical compounds and all sorts of other yawn-inducing topics. Paper stored in absolutely ideal temperature and humidity controlled vaults still continues to turn yellow. Time marches on for all things.
Even museums have a hard time keeping stuff from rotting away. Take a look...
https://archives.cnn.com/2000/US/08/24/pvc.toy.danger/
https://www.sciencenews.org/pages/pdfs/data/2000/158-24/15824-19.pdf
For those of you too lazy to click, check out this snippet: "Some of the most vulnerable new materials are plastics. Museums display them as toys, medical equipment, footwear, inflatable furniture, and more, says Yvonne Shashoua of the National Museum of Denmark. “They’re found in every museum in the world,” she says. Yet many plastics exhibited in museums can change so much chemically that within a decade they start to feel tacky. Many such objects must be taken out of a collection after just 20 years, says Shashoua."
So, what choices do we have?
1. Enjoy the figures. Take them out of the box, look at them, pose them and put them on a shelf where you can see them. Show them to your friends. If you are not to hot, or cold, or humid or dry then your figures are not as well. When they really start to melt, leak, crack and peel don't be shocked.
2. Sell them.