As long as your soil is good and they get enough moisture, they should be fine. I have grown them more than once at that spacing when I wanted shade over a western window.

6 inches is fine. But they'll need outgrow the trellis. I use 8 foot poles for mine. They go straight up the poles. They don't branch out so they don't need 2.5 feet width of support. And please eat the beans. They are not just ornamental. In fact over here they are not even considered as flowers, just as vegetables.

if I pinch the leading stem, won't that encourage lateral growth?

They take a while to branch if the leader is damaged. They aren't really suited for covering width. They're not naturally bushy. Imo it will just become a tangled mess. As you can see it climbs by twining so it prefers to head straight up a pole or string. But you can always try. I've never attempted to grow them as anything but a straight forward pole bean. They're just grown as a common vegetable here.