Thank you all for your responses. And for linking to my video and album. I feel better about this rust now. I'm quite confident it is indeed rust as the bottom vent holes show some rust. But I'm not too worried about it after reading your comments. Also, for what it is worth, the seatstays sound ok when knocking them. I feel like improving the situation anyway, since the frame is bare and I have now a relatively good access through the bottom vent holes and the bigger holes at the seat tube intersection. I have some oxalic and citric acids around. I will have a look about locally available (I'm in Switzerland) products for future protection.
I like the Motorex products which is my go to local manufacturing company for grease and lubricants. I think their 466 fluid might fit the bill, it is their anticorrosion waxy protective film motorex.com/en-us/farmer-forest-garden-line/protection-anticorrosion/fluid-466/ and I can buy it locally. It might make a nice 'frame saver'.