WP Front Scroll Top is a nice button that express scrolls up. Smooth Scroll is a less attractive set of buttons that local- and express-scroll up and local-scroll down. Both work very nicely. But is there anything that does express-scroll down? I can't find anything. Not vital, but hope springs . . .
Express-to-bottom scroll problem solved for all intents and purposes. I discover that Smooth Scroll Page Up/Down offers the option of adjusting the length of the local scroll by percentage of screen. Setting that to something above 100 per cent essentially express-scrolls in either direction–say, 500 per cent scrolls five screens-worth. Not particularly elegant–and the buttons aren’t cute–but it works!