Yes, KWin supports Alt-Esc, but it's not configured by default. To enable it, run systemsettings, Keyboard & Mouse, Global Keyboard Shortcuts. Select KWin, find Lower Window and assign Alt-Esc to it. It does not conflict with Alt-Tab.
Alt-MMB /is/ assigned to Lower Window out of the box.
Oh and if you really like IceWM, you can start a KDE session using IceWM by putting KDE_WM=icewm in your environment.