Got Caps Lock working as the Compose key, so that's great. The answer was to specify xkb_options compose:caps for the keyboard input. But getting the less key instead of the backslash key remains a problem. xev shows "keycode 94 (keysym 0x3c, less)" for the key in question. If only I knew the name of a variant that would immediately change that to backslash.
OK, using "us" layout and "intl" variant the backslash key works as intended, but now I have all those accents to deal with, having to press them twice for them to not combine with the next letter. Gaaah. And always the guesswork involved.