I suspect Part of the condensation problem will also be that you are continually pumping moist air through chamber, bracing roof with Blue water pipe should work, have used it in the past to build mini polly tunnels, jut need to refine design so it is free standing inside air chamber as for Fans I would turn off/remove and seal and use a dehumidifier. The De humidifier you linked to is unlikely to perform at quoted figures at outdoor temperature in winter, so as you say would also need a heater, if you use a desiccant type dehumidifier I believe they are more efficient at lower temperature, but also generate more of their own heat as part of the process, I use one of these here which I find very efficient, if you pipe to external drain then the tank would not need emptying, my one concern would be the drain pipe freezing. I guess you could line with bubble wrap to increase insulation of air chamber and I suppose you could use external drain pipe into a jerry can and just empty once a month or so, once the air inside is dry then the dehumidifier goes into a low power mode. The main annoyance with this dehumidifier is if it looses power it does not auto start so you have to remember manualy start it up, I think there are probably desiccant dehumidifiers that will auto start.
Tim