I don't have the K3565-Z anymore and so I can't try whatever DC-unlocker has published, but using the generic dashboards from other ZTE-devices doesn't work, as the K3565-Z requires special drivers. Actually those differing drivers were the biggest hurdle in the debranding process. On the linked wiki you will also find instructions for the soft debranding, where the flashmemory will just be disabled and the unbranded dashboard will be stored on a microSD-card.
Regarding the promotional price of € 20, I can't give you any information, as it was part of local promotions of some Saturn stores and usually their promotions last for 1-2 weeks.
Anyway I just discovered that O2 also has an interesting offer: They sell a Huawei E161 for € 30. The E161 was released only some weeks ago and it is very tiny - the tinest 3G modem I've seen so far. It's netlocked, but that's not really an issue, if you know how to use eBay. But make sure it's really the Huawei E161 if you buy it in a O2 shop or some other retailer, as some may still have older models in stock. Here's a photo of the E161:
o2online - Prepaid-Surfstick
Regarding the inexistant voice-capability of those data-SIMs and prolongation, I actually don't have an answer. It's definitley impossible to place voice calls with those SIMs - so I hope Vodafone will realize that problem and prolong validiy based on usage and not on recharges.