Rohloff provides very good guarantee so far. I bought a second hand Rohloff in 2009, which failed during my trip in India in 2013. Once back in the Netherlands it was repaired free of charge, all I had to do is deliver it at a dealer (where I did not buy it from), and pick it up when repaired.
I heard that Rohloff is considering a less free guarantee though. People seem to keep on and on cycling with those Rohloff hubs, and malfunctions of very old hubs are increasing. So for newer hubs things stay free, for the older hubs a fee will be asked. Which I think is fair. I was surprised that my service was completely free already.
I never heard of stamped service cards being required. They sell the oil separately, if they don't want people servicing their own hubs they shouldn't support it. It also wouldn't be handy on longer trips to not be permitted to service ones own hub.
Water and mechanics never combine very well, but I don't expect problems from rain.
BTW: to any technical stuff Murphy's law applies: just keep pedalling with that hub and it will fail. It will probably only fail after a very very long time.
What is tiresome of Rohloff is that their company shuts down for two weeks twice per year, once during summer, and once around Christmas. You cannot expect anything from them during that time. They refer to their importers abroad, but India didn't have one, and the Australian one sucked monkey balls.