Great thread. Thank you for documenting!
I am surrounded by red alder (alnus rubra) for hours in every direction here including right outside my window and I've got a ravine below my house where they get chewed on by beavers. My collecting group (posse?) has collected about 130 rubra seedlings in the last couple years. I've kept 3 for myself (one medium and two tiny mame size ones), and most of the rest went to Left Coast Bonsai for further development and eventual sale. Wild red alder seedlings are fun to collect around here because if you find a good site with just the right conditions, they can form really nice flat root systems, as well as trunks with really interesting and twisting initial bends. In the sites where I've collected, they have to fight their way through duff and slash while sitting on a rather shallow bed of soil (before hitting a thicker layer just below) and this gives them some nice characteristics for bonsai.
Looking through your pictures, red and grey alder look remarkably similar when leafless. The roots, the bark, the internodes, the buds, branching angles, etc all look quite close. The difference in foliage is the main giveaway (red alder is a lot more ruffles potato chip like) and I would say I prefer the appearance of the grey alder leaves. My teacher has an arizona alder (a. oblongifolia) which is quite a nice species also (in foliage and also mature bark / twig appearance).
Alder is an awesome species. In the PNW, rubra puts on girth insanely fast especially in a greenhouse. I'm very curious how grey alder would compare to red alder if grown side-by-side in the same conditions here. I also wonder how red alder would compare in your climate.
Between myself and my mentor John, we've messed with dozens of red alders, and one thing you should know is that they respond to both complete and partial defoliation very well (especially in the case of John's alders in a greenhouse, the response is outrageous). I can't speak for grey alder, but the species looks extremely similar and seems to have a similar level of leaping vigor in the mid-season, so you may want to think about giving it a try on one of your trees. We've done it to big ones and small ones and they all respond. Your mileages may vary in shorter growing seasons but the response is quite rapid if there is lots of ambient heat. So hopefully that will help build ramification in grey alder as well.