I have absolutely no first-hand experience of returning a hub to Rohloff but some general remarks might help.
1. Rohloff has a new guarantee. It is not transferable to the second owner but I've never heard of a genuine second owner being refused service.
2. Even beyond the new guarantee, Rohloff generally does not charge the first owner for repairs unless the hub has been abused by failure to maintain it or by unauthorized disassembly. Once more, I haven't heard of second owners being refused, or charged over the top -- and we would have, for sure; people who can afford a Rohloff are pretty articulate and plugged in.
3. A Rohloff gear box isn't a fully sealed device like a car gearbox. It is intended to "mist out" a certain amount of oil, especially if you have filled it to the handbook spec or over (25ml of new oil after cleaning and draining thoroughly). So oil appearing on the gearbox and spokes, or even on the floor below it, is not necessarily cause for returning the gearbox to Rohloff: it depends on how much oil appears. You also want to be certain the puddle is not condensed water (as a newbie Rohloff owner a puddle of condensed water caused me a bit of a panic...).
4. Before you even approach Rohloff, you should ascertain that the consumables in the drivetrain are not responsible for the problem. You have three examples of the same problem. Did you fit a whole bunch of chains new at the same time? You should check that the chains are still serviceable and inspect the sprockets closely for wear. To me it sounds like both chain and socket are worn on each of the bikes with the problem, and that back-pedalling merely engages a less worn part of both which works at certain velocities. Also, while a Rohloff chain is intended to be fitted quite slack (about a centimeter of vertical movement), if the chain is too slack, perhaps from wear, it should be tightened by moving the hub back in the horizontal slots or adjusting the eccentric bottom bracket, and if these are at the end of their adjustment, taking out a link.
5. Rohloff presumably have their ways of telling whether a hub has been serviced at the specified intervals of 5K. But I have not heard of proof being demanded and Rohloff are aware that world tourers and others sometimes go twice that distance before the service kit catches up with them.
6. What you're in fact more likely to have to prove is that you came by the used Rohloff gearboxes legally. Rohloff is quite clear that they refuse service to any gearbox that has the serial number sticker removed (and the serial is engraved on the case in the current series). If you have the serial number stickers still attached, and receipts for your gearboxes, it might be smart to photocopy proof of ownership and supply it before being asked. (I know this sounds offensive, but I have considerable experience of doing business with Germans. A German would do this and consider it proper. Hell, a German lady, declining my invitation to dinner, included a doctor's certificate to prove she was really too ill to go out...)
7. I don't know where you live, but unless you live next door to Rohloff, it could be expensive to send three wheels or even three gearboxes cut out of the wheels to Rohloff, and you'll kick yourself if you then discover it's a matter of replacing a paper filter on each (just for the sake of an example -- I'm not suggesting a new paper filter is what is required) and the labour charges amount to twenty spondulicks and the parts are so trivial they're free -- but you're out of hundreds for carriage. Go to the dealer, explain, ask him for his advice. People love giving advice.
Good luck
