So it is not necessary to buy mountaineering/randonee boots to use DIN-standard randonee bindings?
No, but you do have to adjust the height of the toe piece to be higher for the thicker alpine boot. Other than that, it's step in compatibility (assuming the right boot sole length adjustment, which is also exceedingly easy on most AT bindings).
How do you find skinning up in full-fledged alpine boots? I realize there's a tradeoff between weight and flexibility on the climb and downhill performance.
It's a pain in the ass. A set of alpine boots can weight twice as much as a pair of AT boots. However, for just starting out, it's definitely doable. But it isn't only the weight that is a drawback, it's the fact that with alpine boots, you're always forced to lean forward, where AT boots, in walk mode, have a mobile cuff.
Also curious anyone's experience with Alpine Trekkers. From what I've read the weight and height are pretty significant drawbacks but they might work for me for a start, since my initial "BC" is likely to be a NELSAP hill and other local terrain with generally no more than 200' vertical. I do like the idea of being able to try this out for an initial investment of < $200 rather than much more for AT bindings and skis to mount them on.
Weight and height are not big deals because of the Trekkers themselves. It's the boots that add weight. If your alpine bindings are not tall, you won't be any higher than you'd be on a set of Freerides. I used my Trekkers on two or three average length tours (~3000' vert) and found them to be usable. You can find them for much cheaper than $180 because most people buy them and never tour again and sell them, or buy them and then buy AT setups and sell them. I've kept mine around for my friends who are just getting into touring or just to have in case I'm on my alpine gear and I want to do some slack country. They're perfect for that.
The other thing to consider besides the initial cost is you'll likely lose far less money buying and then reselling a pair of trekkers than you would buying a full AT setup, possibly learning you don't like touring, and then turning around and trying to sell all of it. Especially if you have a friend or two that are lagging behind you, use the trekkers and then sell them to one of them, or keep them like I did. A lot of people don't like them, but I think they fit their niche perfectly, assuming you have that niche to fill. If you do buy them, though, loctite all the fasteners.