In flwm, as you may have found out, Alt + Tab brings up the system menu (same as right-clicking on the background).  When that is open, the next window in the sequence is highlighted.  So you can just press enter to raise and activate that window.  So Alt + Tab in something like windows can be done by Alt + Tab, (release), Enter in flwm.  Maybe not as convenient, but not bad either with a little practice
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Mike L.