In a wider sense that means a lens which barrel can be pushed into the camera body for transport.
yes the CL accepts M lenses many have been stripped of their lens
the 40mm Voightlander is cheaper available single or multi coated and a stop faster, a generation later from the glass catalogue as well
except early wides like super angelon and Elmarit 28mm type I, collapsibles and some LTM lenses like J12 all of which will cause mechanical damage.
the rangefinder has a limited base length so fast longs difficult to nail focus.