First, a disclaimer: I haven't really used 2.7 yet, so I'm speaking "theoretically "here.
Not sure what you really ask here. DevMode does 2 things: launch CodeServer with -launcherDir <warDir> and start an embedded Jetty serving <warDir> as a webapp.
With the -noserver flag, DevMode is thus equivalent to launching CodeServer with -launcherDir <warDir>, and you're supposed to launch your own servlet container to serve <warDir> as a webapp.
So how is that different from launching CodeServer and using the bookmarklets? The *.cache.js is served from CodeServer in all cases, as are the source maps.
So theoretically there's no reason using DevMode with -noserver to be the cause of source maps not showing.