Hi Nick,
I overwinter all my alpines outdoors, either unprotected or in a frame. I think the elements do them good, by and large as many species need a prolonged cold period to stimulate growth and / or flowering in spring.
HOWEVER, many species may need protection from winter wet (especially Himalayan species). The extent to which this is necessary depends on how wet a winter we get, the species concerned, the size of the seedlings (as you say, small ones can rot off more quickly), and the drainage of your compost (I generally put a layer of perlite at the bottom of all my pots with smaller seedlings in). To be honest most of the plants in the photos you posted look big enough to plant out, never mind overwinter!
Some possible solutions for small seedlings:
- Put them in a frame with the lid on but partly open (so some rain can blow in and they don't dry out) - this is the best thing to do I think.
- Raise the pots / trays up to ensure better drainage (you often get a film of water under them)
- Repot into a more freely draining mix (though probably not great to do this now)
Hope this helps - don't think you have too much to worry about though.
Best, Tristan