Good news: you might not need to add a radon tank to the hot tub you already own. The control group for [2] was people who did not do any hot spring bathing at all during an average week, so their methodology does not explore the significance of the radon itself.
The interesting thing is that they didn't find a higher incidence of lung, esophageal, and other soft-tissue cancers in elderly folks who bath in radon springs more than once a week.
The blood pressure reduction probably has more to do with heat exposure than with radon itself. Seems to me that regular exposure to a normal sauna or hot spring would probably elicit similar results.
People don't die of cancer instantly -- in many cases, it's pretty indolent, and "alive with cancer"/"in remission" are fairly common statuses. This would surely be picked up in the respondent data.
If I were dying of cancer, odds are good that I would have ended up in the control group for not being especially proactive about visiting the hot springs. The authors of [2] explain this limitation themselves:
"Some study limitations should be considered. ... healthier people may have been more likely to bathe in the radon hot springs."
